Make sure that RGM Matrix is checked properly

users/drashna/rules.mk

@@ -33,6 +33,7 @@ ifeq ($(strip $(RGBLIGHT_ENABLE)), yes)
     endif
 endif
 
+RGB_MATRIX_ENABLE ?= no
 ifneq ($(strip $(RGB_MATRIX_ENABLE)), no)
     SRC += rgb_stuff.c
 endif