mkbootimg: turn on -Werror

- Deal with signedness issues

Change-Id: I61211165eb4ce0fa9a2edeebb3f97fb0ffc62d34
diff --git a/mkbootimg/Android.mk b/mkbootimg/Android.mk
index 2a97c26..0c9b0c6 100644
--- a/mkbootimg/Android.mk
+++ b/mkbootimg/Android.mk
@@ -4,6 +4,7 @@
 
 LOCAL_SRC_FILES := mkbootimg.c
 LOCAL_STATIC_LIBRARIES := libmincrypt
+LOCAL_CFLAGS := -Werror
 
 LOCAL_MODULE := mkbootimg