build_keyboard.mk: Restore UCIS_ENABLE support

During the build system refactor, support for enabling UCIS seems to
have been lost. This little patch adds that back, so that keymaps using
UCIS can be compiled again.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

build_keyboard.mk

@@ -148,6 +148,11 @@ ifeq ($(strip $(AUDIO_ENABLE)), yes)
 	SRC += $(QUANTUM_DIR)/audio/luts.c
 endif
 
+ifeq ($(strip $(UCIS_ENABLE)), yes)
+	OPT_DEFS += -DUCIS_ENABLE
+	UNICODE_ENABLE = yes
+endif
+
 ifeq ($(strip $(UNICODE_ENABLE)), yes)
     OPT_DEFS += -DUNICODE_ENABLE
 	SRC += $(QUANTUM_DIR)/process_keycode/process_unicode.c