add json_object_object_length
diff --git a/json_object.h b/json_object.h
index 0ff6584..336ce96 100644
--- a/json_object.h
+++ b/json_object.h
@@ -215,6 +215,8 @@
  */
 extern struct lh_table* json_object_get_object(struct json_object *obj);
 
+extern int json_object_object_length(struct json_object* obj);
+
 /** Add an object field to a json_object of type json_type_object
  *
  * The reference count will *not* be incremented. This is to make adding