payload in Adafruit_MQTT publish should be const

[flavio-fernandes]

The Method:
bool Adafruit_MQTT_Publish::publish(uint8_t *payload, uint16_t bLen)

should be

bool Adafruit_MQTT_Publish::publish(const uint8_t *payload, uint16_t bLen)

since payload is not modified in the implementation.

Proposed fix in PR #114