Fix compiler warnings in frameworks/av/media/ndk.

Bug: 26936282
Change-Id: I86b32ef5a7e6b18974cbe4a885291acec2b2eb5f
diff --git a/media/ndk/Android.mk b/media/ndk/Android.mk
index 8dbb291..f287761 100644
--- a/media/ndk/Android.mk
+++ b/media/ndk/Android.mk
@@ -40,6 +40,8 @@
 
 LOCAL_CFLAGS += -fvisibility=hidden -D EXPORT='__attribute__ ((visibility ("default")))'
 
+LOCAL_CFLAGS += -Werror
+
 LOCAL_SHARED_LIBRARIES := \
     libbinder \
     libmedia \