minui: Do not overwrite previously set CFLAGS

Change-Id: Ifa5bcffa12c045cb629974684882f1e0f56dcce6
diff --git a/minui/Android.mk b/minui/Android.mk
index e359887..8df5a49 100644
--- a/minui/Android.mk
+++ b/minui/Android.mk
@@ -62,8 +62,8 @@
     libpng \
     libbase
 
-LOCAL_CFLAGS := -Werror -std=c++14
-LOCAL_C_INCLUDES := $(LOCAL_PATH)/include
+LOCAL_CFLAGS += -Werror -std=c++14
+LOCAL_C_INCLUDES += $(LOCAL_PATH)/include
 LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/include
 
 LOCAL_MODULE := libminui