Clean up our final native code warnings, and turn on -Werror.

Change-Id: I54bb29e357812b089e83f3a2a831c80f5114a27a
diff --git a/NativeCode.mk b/NativeCode.mk
index d448877..6caf79c 100644
--- a/NativeCode.mk
+++ b/NativeCode.mk
@@ -85,7 +85,7 @@
 
 include $(CLEAR_VARS)
 
-LOCAL_CFLAGS += -Wall -Wextra
+LOCAL_CFLAGS += -Wall -Wextra -Werror
 
 ifeq ($(TARGET_ARCH),arm)
 # Ignore "note: the mangling of 'va_list' has changed in GCC 4.4"