Add extern to json_object_set_serializer so that it gets exported (Windows fix)
diff --git a/json_object.h b/json_object.h
index 6270309..df958bf 100644
--- a/json_object.h
+++ b/json_object.h
@@ -192,7 +192,7 @@
* @param userdata an optional opaque cookie
* @param user_delete an optional function from freeing userdata
*/
-void json_object_set_serializer(json_object *jso,
+extern void json_object_set_serializer(json_object *jso,
json_object_to_json_string_fn to_string_func,
void *userdata,
json_object_delete_fn *user_delete);