periph/rtt: add missing std header
periph/rtt: add missing std header

The periph/rtt.h uses `uintX_t` typedefs but misses the required
standard header file, namely `stdint.h`, which is added here.

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