Issue #263: add const so the prototype for json_object_object_add_ex() matches the definition.

json_object.h

@@ -385,8 +385,10 @@ extern int json_object_object_add(struct json_object* obj, const char *key,
  * @param opts process-modifying options. To specify multiple options, use 
  *             arithmetic or (OPT1|OPT2)
  */
-extern int json_object_object_add_ex(struct json_object* obj, const char *key,
-				   struct json_object *val, const unsigned opts);
+extern int json_object_object_add_ex(struct json_object* obj,
+				const char *const key,
+				struct json_object *const val,
+				const unsigned opts);
 
 /** Get the json_object associate with a given object field.
  *