MQTT Client should compile with only Ethernet enabled

[nseidle]

Using the following compile guard setup

  // #define COMPILE_BT       // Comment out to remove Bluetooth functionality
  // #define COMPILE_WIFI     // Comment out to remove WiFi functionality
  #define COMPILE_ETHERNET // Comment out to remove Ethernet (W5500) support

We should be able to compile but we cannot. the MQTT_Client.ino throws errors because it requires WiFi. Does ESP32 really require the adding of the WiFi library just so we can use the built in MQTT client over ethernet?

[nseidle]

This is in release v1.3.