Enable -Werror for external/libmpeg2.

Bug: 28027910
Change-Id: I49efa732f29d199ecf5ed8fb3b8e7563192cab4d
diff --git a/decoder.mk b/decoder.mk
index c1e5861..bf3fa03 100644
--- a/decoder.mk
+++ b/decoder.mk
@@ -10,7 +10,7 @@
 LOCAL_MODULE_CLASS := STATIC_LIBRARIES
 
 LOCAL_CFLAGS += -D_LIB -DMULTICORE -fPIC
-LOCAL_CFLAGS += -O3 -DANDROID
+LOCAL_CFLAGS += -O3 -DANDROID -Werror
 
 LOCAL_C_INCLUDES := $(LOCAL_PATH)/decoder $(LOCAL_PATH)/common