Change topics concatenation method

Correct regression introduced by #503
Nevertheless this method needs to be optimized to avoid the use of string.

main/ZgatewayPilight.ino

@@ -74,7 +74,7 @@ void MQTTtoPilight(char * topicOri, JsonObject& Pilightdata) {
 
   int result = 0;
   
-  if (strstr(topicOri,catToMainTopic(subjectMQTTtoPilight)) != NULL){
+  if (strstr(topicOri,(char *)catToMainTopic(subjectMQTTtoPilight).c_str()) != NULL){
     trc(F("MQTTtoPilight json data analysis"));
     const char * message = Pilightdata["message"];
     const char * protocol = Pilightdata["protocol"];