Changed callback type.

src/PubSubClient.cpp

@@ -570,7 +570,7 @@ PubSubClient& PubSubClient::setServer(const char * domain, uint16_t port) {
     return *this;
 }
 
-PubSubClient& PubSubClient::setCallback(void(*callback)(char*,uint8_t*,unsigned int)){
+PubSubClient& PubSubClient::setCallback(MQTT_CALLBACK_SIGNATURE) {
     this->callback = callback;
     return *this;
 }