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'.

sys/shell/commands/Makefile

@@ -70,7 +70,7 @@ ifneq (,$(filter cord_ep,$(USEMODULE)))
   SRC += sc_cord_ep.c
 endif
 
-ifneq (,$(filter periph_rtc,$(FEATURES_PROVIDED)))
+ifneq (,$(filter periph_rtc,$(USEMODULE)))
   SRC += sc_rtc.c
 endif