make: fix gnrc_slip dependency

Makefile.dep

@@ -353,6 +353,10 @@ ifneq (,$(filter gnrc_pktdump,$(USEMODULE)))
   USEMODULE += od
 endif
 
+ifneq (,$(filter gnrc_slip,$(USEMODULE)))
+  FEATURES_REQUIRED += periph_uart
+endif
+
 ifneq (,$(filter od,$(USEMODULE)))
   USEMODULE += fmt
 endif

sys/Makefile.include

@@ -43,10 +43,6 @@ ifneq (,$(filter cpp11-compat,$(USEMODULE)))
   export UNDEF += $(BINDIR)/cpp11-compat/cppsupport.o
 endif
 
-ifneq (,$(filter gnrc_slip,$(USEMODULE)))
-  FEATURES_REQUIRED += periph_uart
-endif
-
 ifneq (,$(filter embunit,$(USEMODULE)))
   ifeq ($(OUTPUT),XML)
     CFLAGS += -DOUTPUT=OUTPUT_XML