kitchen_pilot_wire_control.ino: add a message on inactivity

soft/kitchen/kitchen_pilot_wire_control/kitchen_pilot_wire_control.ino

@@ -51,6 +51,7 @@ void shutdown_heater(void *dummy)
 {
   // if we didn't receive a message for an hour,
   // shutdown the heater
+  Serial.println("No MQTT message for one hour, switching off.");
   heater_off(heating_pin);
 }