usermods: Fix MQTT checks

- Check after including wled.h
- Use WLED_DISABLE_MQTT instead of WLED_ENABLE_MQTT

usermods/Si7021_MQTT_HA/Si7021_MQTT_HA.cpp

@@ -1,7 +1,3 @@
-#ifndef WLED_ENABLE_MQTT
-#error "This user mod requires MQTT to be enabled."
-#endif
-
 // this is remixed from usermod_v2_SensorsToMqtt.h (sensors_to_mqtt usermod)
 // and usermod_multi_relay.h (multi_relay usermod)
 
@@ -9,6 +5,10 @@
 #include <Adafruit_Si7021.h>
 #include <EnvironmentCalculations.h> // EnvironmentCalculations::HeatIndex(), ::DewPoint(), ::AbsoluteHumidity()
 
+#ifdef WLED_DISABLE_MQTT
+#error "This user mod requires MQTT to be enabled."
+#endif
+
 Adafruit_Si7021 si7021;
 
 class Si7021_MQTT_HA : public Usermod