tests/at: add missing stdio.h include
tests/at: add missing stdio.h include

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