tools: show diff for headerguards check on failure
tools: show diff for headerguards check on failure

    fixes #8737

x3ro/RIOT,kaspar030/RIOT,OTAkeys/RIOT,avmelnikoff/RIOT,RIOT-OS/RIOT,biboc/RIOT,smlng/RIOT,x3ro/RIOT,ant9000/RIOT,x3ro/RIOT,mfrey/RIOT,miri64/RIOT,aeneby/RIOT,jasonatran/RIOT,toonst/RIOT,neiljay/RIOT,avmelnikoff/RIOT,BytesGalore/RIOT,RIOT-OS/RIOT,mfrey/RIOT,A-Paul/RIOT,miri64/RIOT,smlng/RIOT,kYc0o/RIOT,OTAkeys/RIOT,BytesGalore/RIOT,rfuentess/RIOT,aeneby/RIOT,rfuentess/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kbumsik/RIOT,ant9000/RIOT,RIOT-OS/RIOT,mtausig/RIOT,RIOT-OS/RIOT,rfuentess/RIOT,toonst/RIOT,yogo1212/RIOT,authmillenon/RIOT,BytesGalore/RIOT,josephnoir/RIOT,OlegHahm/RIOT,jasonatran/RIOT,gebart/RIOT,kaspar030/RIOT,avmelnikoff/RIOT,authmillenon/RIOT,yogo1212/RIOT,kYc0o/RIOT,biboc/RIOT,josephnoir/RIOT,yogo1212/RIOT,kYc0o/RIOT,lazytech-org/RIOT,kbumsik/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,smlng/RIOT,cladmi/RIOT,smlng/RIOT,lazytech-org/RIOT,authmillenon/RIOT,lazytech-org/RIOT,OTAkeys/RIOT,josephnoir/RIOT,A-Paul/RIOT,cladmi/RIOT,toonst/RIOT,kbumsik/RIOT,x3ro/RIOT,OTAkeys/RIOT,avmelnikoff/RIOT,BytesGalore/RIOT,basilfx/RIOT,OlegHahm/RIOT,toonst/RIOT,yogo1212/RIOT,OlegHahm/RIOT,mtausig/RIOT,cladmi/RIOT,miri64/RIOT,neiljay/RIOT,biboc/RIOT,gebart/RIOT,ant9000/RIOT,mtausig/RIOT,aeneby/RIOT,A-Paul/RIOT,kYc0o/RIOT,cladmi/RIOT,gebart/RIOT,josephnoir/RIOT,basilfx/RIOT,biboc/RIOT,authmillenon/RIOT,basilfx/RIOT,mtausig/RIOT,basilfx/RIOT,mtausig/RIOT,x3ro/RIOT,rfuentess/RIOT,miri64/RIOT,gebart/RIOT,mfrey/RIOT,A-Paul/RIOT,aeneby/RIOT,yogo1212/RIOT,mfrey/RIOT,kaspar030/RIOT,biboc/RIOT,ant9000/RIOT,yogo1212/RIOT,toonst/RIOT,kbumsik/RIOT,BytesGalore/RIOT,lazytech-org/RIOT,jasonatran/RIOT,authmillenon/RIOT,lazytech-org/RIOT,mfrey/RIOT,kYc0o/RIOT,miri64/RIOT,rfuentess/RIOT,avmelnikoff/RIOT,neiljay/RIOT,kaspar030/RIOT,neiljay/RIOT,josephnoir/RIOT,kaspar030/RIOT,OlegHahm/RIOT,kbumsik/RIOT,gebart/RIOT,aeneby/RIOT,A-Paul/RIOT,basilfx/RIOT,authmillenon/RIOT,OTAkeys/RIOT,cladmi/RIOT,neiljay/RIOT,ant9000/RIOT,smlng/RIOT