Make sure to include the "*" on function pointer arguments to avoid a warnings from VS2015. See also PR#384.

json_object.h

@@ -324,7 +324,7 @@ JSON_EXPORT void json_object_set_userdata(json_object *jso, void *userdata,
  * @param user_delete an optional function from freeing userdata
  */
 JSON_EXPORT void json_object_set_serializer(json_object *jso,
-	json_object_to_json_string_fn to_string_func,
+	json_object_to_json_string_fn *to_string_func,
 	void *userdata,
 	json_object_delete_fn *user_delete);