Change topics concatenation method

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

main/ZgatewayIR.ino

@@ -274,7 +274,7 @@ void IRtoMQTT(){
 #ifdef jsonReceiving
   void MQTTtoIR(char * topicOri, JsonObject& IRdata) {
 
-     if (strstr(topicOri,catToMainTopic(subjectMQTTtoIR)) != NULL){ 
+     if (strstr(topicOri,(char *)catToMainTopic(subjectMQTTtoIR).c_str()) != NULL){ 
       trc(F("MQTTtoIR json"));
       uint64_t data = IRdata["value"];
       const char * raw = IRdata["raw"];