sys/shell: select sc_rtc on USEMODULE
sys/shell: select sc_rtc on USEMODULE

sc_rtc.c should only be compiled if periph_rtc module is actually used.

In practice there was not linking error when PERIPH_OPTIONAL|_REQUIRED
was not set as shell_commands hides calling the functions with
'#ifdef MODULE_PERIPH_RTC'.

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