[Keymap] Fix RGB_MATRIX_ENABLE check in users/konstantin/rules.mk (#5832)

users/konstantin/rules.mk

@@ -2,6 +2,7 @@ SRC += konstantin.c
 ifeq ($(strip $(RGBLIGHT_ENABLE)), yes)
     SRC += rgb.c
 endif
+RGB_MATRIX_ENABLE ?= no
 ifneq ($(strip $(RGB_MATRIX_ENABLE)), no)
     SRC += rgb.c
 endif