Fix warnings, re-enable -Werror.

Change-Id: Ic6b1a10f46f980f9a9d8d130600419a792703c44
diff --git a/media/img_utils/src/Android.mk b/media/img_utils/src/Android.mk
index 29c011f..80893be 100644
--- a/media/img_utils/src/Android.mk
+++ b/media/img_utils/src/Android.mk
@@ -46,6 +46,7 @@
 LOCAL_CFLAGS += \
   -Wall \
   -Wextra \
+  -Werror \
   -fvisibility=hidden
 
 ifneq ($(filter userdebug eng,$(TARGET_BUILD_VARIANT)),)