Suppress benign clang warning.

To unblock continuous clang/llvm builds.

BUG: 21337624
Change-Id: I38e5d11fd3c44f50430a3a8519c5071674b3df9f

libs/hwui/Android.common.mk

@@ -1,8 +1,10 @@
 # getConfig in external/skia/include/core/SkBitmap.h is deprecated.
 # Allow Gnu extension: in-class initializer of static 'const float' member.
+# DeferredLayerUpdater.h: private field 'mRenderThread' is not used.
 LOCAL_CLANG_CFLAGS += \
     -Wno-deprecated-declarations \
-    -Wno-gnu-static-float-init
+    -Wno-gnu-static-float-init \
+    -Wno-unused-private-field
 
 LOCAL_SRC_FILES := \
     font/CacheTexture.cpp \