sys/cpp11-compat: Add braces around mutex initializer
sys/cpp11-compat: Add braces around mutex initializer

Fixes Clang warning/error:

    In file included from .../riot/sys/cpp11-compat/thread.cpp:26:
    In file included from .../riot/sys/cpp11-compat/include/riot/thread.hpp:39:
    .../riot/sys/cpp11-compat/include/riot/mutex.hpp:47:45: error:
     suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
      inline constexpr mutex() noexcept : m_mtx{0} {}
                                                ^
                                                {}
    1 error generated.

backenklee/RIOT,adjih/RIOT,rfuentess/RIOT,neumodisch/RIOT,avmelnikoff/RIOT,A-Paul/RIOT,katezilla/RIOT,biboc/RIOT,josephnoir/RIOT,ks156/RIOT,jasonatran/RIOT,kerneltask/RIOT,mfrey/RIOT,gautric/RIOT,gebart/RIOT,adrianghc/RIOT,cladmi/RIOT,mfrey/RIOT,immesys/RiSyn,neiljay/RIOT,miri64/RIOT,x3ro/RIOT,wentaoshang/RIOT,alignan/RIOT,d00616/RIOT,TobiasFredersdorf/RIOT,tfar/RIOT,roberthartung/RIOT,ant9000/RIOT,jremmert-phytec-iot/RIOT,neumodisch/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,Josar/RIOT,alignan/RIOT,adjih/RIOT,thomaseichinger/RIOT,ks156/RIOT,LudwigKnuepfer/RIOT,kaspar030/RIOT,TobiasFredersdorf/RIOT,thomaseichinger/RIOT,dailab/RIOT,brettswann/RIOT,kaleb-himes/RIOT,jremmert-phytec-iot/RIOT,authmillenon/RIOT,hamilton-mote/RIOT-OS,alignan/RIOT,jbeyerstedt/RIOT-OTA-update,LudwigOrtmann/RIOT,syin2/RIOT,OlegHahm/RIOT,roberthartung/RIOT,hamilton-mote/RIOT-OS,wentaoshang/RIOT,basilfx/RIOT,gautric/RIOT,ks156/RIOT,TobiasFredersdorf/RIOT,OTAkeys/RIOT,dkm/RIOT,altairpearl/RIOT,immesys/RiSyn,lazytech-org/RIOT,immesys/RiSyn,kYc0o/RIOT,LudwigKnuepfer/RIOT,Hyungsin/RIOT-OS,RIOT-OS/RIOT,x3ro/RIOT,rajma996/RIOT,jremmert-phytec-iot/RIOT,tfar/RIOT,thomaseichinger/RIOT,LudwigOrtmann/RIOT,hamilton-mote/RIOT-OS,roberthartung/RIOT,aeneby/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,katezilla/RIOT,Ell-i/RIOT,tfar/RIOT,plushvoxel/RIOT,LudwigOrtmann/RIOT,basilfx/RIOT,kerneltask/RIOT,altairpearl/RIOT,kb2ma/RIOT,BytesGalore/RIOT,kbumsik/RIOT,kaspar030/RIOT,d00616/RIOT,adjih/RIOT,plushvoxel/RIOT,wentaoshang/RIOT,d00616/RIOT,OTAkeys/RIOT,backenklee/RIOT,dkm/RIOT,jremmert-phytec-iot/RIOT,alignan/RIOT,brettswann/RIOT,gebart/RIOT,kYc0o/RIOT,miri64/RIOT,authmillenon/RIOT,syin2/RIOT,gautric/RIOT,rfuentess/RIOT,josephnoir/RIOT,jfischer-phytec-iot/RIOT,smlng/RIOT,mtausig/RIOT,tfar/RIOT,OlegHahm/RIOT,kbumsik/RIOT,toonst/RIOT,toonst/RIOT,rfuentess/RIOT,kYc0o/RIOT,LudwigOrtmann/RIOT,ant9000/RIOT,beurdouche/RIOT,kbumsik/RIOT,beurdouche/RIOT,neiljay/RIOT,neumodisch/RIOT,mtausig/RIOT,lazytech-org/RIOT,BytesGalore/RIOT,LudwigOrtmann/RIOT,immesys/RiSyn,RIOT-OS/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,cladmi/RIOT,jasonatran/RIOT,beurdouche/RIOT,kaleb-himes/RIOT,jasonatran/RIOT,mtausig/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,thomaseichinger/RIOT,cladmi/RIOT,gebart/RIOT,neiljay/RIOT,OlegHahm/RIOT,dailab/RIOT,avmelnikoff/RIOT,A-Paul/RIOT,adjih/RIOT,kaspar030/RIOT,wentaoshang/RIOT,mtausig/RIOT,toonst/RIOT,gebart/RIOT,gebart/RIOT,basilfx/RIOT,kbumsik/RIOT,lazytech-org/RIOT,yogo1212/RIOT,BytesGalore/RIOT,LudwigKnuepfer/RIOT,neumodisch/RIOT,Ell-i/RIOT,kaspar030/RIOT,kb2ma/RIOT,syin2/RIOT,backenklee/RIOT,miri64/RIOT,dkm/RIOT,jbeyerstedt/RIOT-OTA-update,LudwigKnuepfer/RIOT,kbumsik/RIOT,beurdouche/RIOT,wentaoshang/RIOT,immesys/RiSyn,mfrey/RIOT,jbeyerstedt/RIOT-OTA-update,brettswann/RIOT,A-Paul/RIOT,brettswann/RIOT,d00616/RIOT,kYc0o/RIOT,yogo1212/RIOT,BytesGalore/RIOT,authmillenon/RIOT,kb2ma/RIOT,hamilton-mote/RIOT-OS,adrianghc/RIOT,kerneltask/RIOT,syin2/RIOT,Hyungsin/RIOT-OS,jasonatran/RIOT,gautric/RIOT,rajma996/RIOT,plushvoxel/RIOT,authmillenon/RIOT,katezilla/RIOT,aeneby/RIOT,yogo1212/RIOT,cladmi/RIOT,x3ro/RIOT,adrianghc/RIOT,smlng/RIOT,A-Paul/RIOT,roberthartung/RIOT,mfrey/RIOT,immesys/RiSyn,jfischer-phytec-iot/RIOT,rfuentess/RIOT,jremmert-phytec-iot/RIOT,altairpearl/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,mtausig/RIOT,x3ro/RIOT,lazytech-org/RIOT,tfar/RIOT,biboc/RIOT,jbeyerstedt/RIOT-OTA-update,Josar/RIOT,Ell-i/RIOT,x3ro/RIOT,brettswann/RIOT,LudwigOrtmann/RIOT,neumodisch/RIOT,katezilla/RIOT,Ell-i/RIOT,josephnoir/RIOT,dkm/RIOT,thomaseichinger/RIOT,beurdouche/RIOT,jfischer-phytec-iot/RIOT,miri64/RIOT,avmelnikoff/RIOT,smlng/RIOT,altairpearl/RIOT,yogo1212/RIOT,plushvoxel/RIOT,alignan/RIOT,ant9000/RIOT,dailab/RIOT,kb2ma/RIOT,miri64/RIOT,jfischer-phytec-iot/RIOT,toonst/RIOT,biboc/RIOT,yogo1212/RIOT,biboc/RIOT,OTAkeys/RIOT,cladmi/RIOT,josephnoir/RIOT,wentaoshang/RIOT,roberthartung/RIOT,Josar/RIOT,toonst/RIOT,adrianghc/RIOT,TobiasFredersdorf/RIOT,neiljay/RIOT,dailab/RIOT,OTAkeys/RIOT,altairpearl/RIOT,kerneltask/RIOT,smlng/RIOT,dkm/RIOT,RIOT-OS/RIOT,d00616/RIOT,OlegHahm/RIOT,kaleb-himes/RIOT,mfrey/RIOT,LudwigKnuepfer/RIOT,kerneltask/RIOT,TobiasFredersdorf/RIOT,brettswann/RIOT,dailab/RIOT,Hyungsin/RIOT-OS,foss-for-synopsys-dwc-arc-processors/RIOT,Josar/RIOT,backenklee/RIOT,OlegHahm/RIOT,rajma996/RIOT,yogo1212/RIOT,ks156/RIOT,gautric/RIOT,syin2/RIOT,aeneby/RIOT,d00616/RIOT,rajma996/RIOT,kYc0o/RIOT,biboc/RIOT,jbeyerstedt/RIOT-OTA-update,aeneby/RIOT,avmelnikoff/RIOT,aeneby/RIOT,adjih/RIOT,kaleb-himes/RIOT,backenklee/RIOT,smlng/RIOT,Hyungsin/RIOT-OS,authmillenon/RIOT,jasonatran/RIOT,basilfx/RIOT,kb2ma/RIOT,Hyungsin/RIOT-OS,neumodisch/RIOT,jremmert-phytec-iot/RIOT,A-Paul/RIOT,hamilton-mote/RIOT-OS,BytesGalore/RIOT,neiljay/RIOT,katezilla/RIOT,kaleb-himes/RIOT,rajma996/RIOT,rfuentess/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,ant9000/RIOT,altairpearl/RIOT,ant9000/RIOT,Ell-i/RIOT,Josar/RIOT,avmelnikoff/RIOT,plushvoxel/RIOT,rajma996/RIOT,ks156/RIOT,adrianghc/RIOT,basilfx/RIOT,jfischer-phytec-iot/RIOT,kaspar030/RIOT