Add hook for mqtt publish event

src/mod_mqtt.erl

@@ -128,6 +128,7 @@ publish({_, S, _} = USR, Pkt, ExpiryTime) ->
         allow ->
             case retain(USR, Pkt, ExpiryTime) of
                 ok ->
+		    ejabberd_hooks:run(mqtt_publish, S, [USR, Pkt, ExpiryTime]),
                     Mod = gen_mod:ram_db_mod(S, ?MODULE),
                     route(Mod, S, Pkt, ExpiryTime);
                 {error, _} = Err ->