Fix compiler warning in media/mtp.

BUG=27152673

Change-Id: I0a5a7f2005bd76acf4d09353305b66af3fd29e4b
diff --git a/media/mtp/Android.mk b/media/mtp/Android.mk
index 3af0956..cb7e4aa 100644
--- a/media/mtp/Android.mk
+++ b/media/mtp/Android.mk
@@ -37,7 +37,7 @@
 
 LOCAL_MODULE:= libmtp
 
-LOCAL_CFLAGS := -DMTP_DEVICE -DMTP_HOST
+LOCAL_CFLAGS := -DMTP_DEVICE -DMTP_HOST -Wall -Wextra -Werror
 
 LOCAL_SHARED_LIBRARIES := libutils libcutils liblog libusbhost libbinder