Add define for MQTT_MAX_PACKET_SIZE

[tbnobody]

Instead of editing the pubsubclient.h file you could just change the code from:

#ifdef USE_MQTT
  #include <PubSubClient.h>                 // MQTT
#endif  // USE_MQTT

to:

#ifdef USE_MQTT
  #define MQTT_MAX_PACKET_SIZE 400
  #include <PubSubClient.h>                 // MQTT
#endif  // USE_MQTT