Add missing art_cflags.

Change-Id: Ie546026f7377b7d229da65f3a6e0a4124a97742a
diff --git a/build/Android.common.mk b/build/Android.common.mk
index 4bf0319..9e4bc67 100644
--- a/build/Android.common.mk
+++ b/build/Android.common.mk
@@ -72,6 +72,7 @@
 
 ifeq ($(ART_USE_LLVM_COMPILER),true)
 art_cflags := \
+	$(art_cflags) \
 	-DART_USE_LLVM_COMPILER=1
 endif