Fixing compile-error when MQTT isn't activated

src/Mqtt.cpp

@@ -179,12 +179,14 @@ bool publishMqtt(const char *topic, uint32_t payload, bool retained) {
 
 // Cyclic posting of WiFi-signal-strength
 void Mqtt_PostWiFiRssi(void) {
+    #ifdef MQTT_ENABLE
         static uint32_t lastMqttRssiTimestamp = 0;
 
         if (!lastMqttRssiTimestamp || (millis() - lastMqttRssiTimestamp >= 60000)) {
             lastMqttRssiTimestamp = millis();
             publishMqtt((char *) FPSTR(topicWiFiRssiState), Wlan_GetRssi(), false);
         }
+    #endif
 }
 
 /* Cyclic posting via MQTT that ESP is still alive. Use case: when ESPuino is switched off, it will post via