`-Wvla` compiler warnings

[zfields]

~/Development/Arduino/libraries/ArduinoMqttClient/src/MqttClient.cpp: In member function 'int MqttClient::subscribe(const char*, uint8_t)':
~/Development/Arduino/libraries/ArduinoMqttClient/src/MqttClient.cpp:333:43: warning: variable length array 'packetBuffer' is used [-Wvla]
   uint8_t packetBuffer[5 + remainingLength];
                                           ^
~/Development/Arduino/libraries/ArduinoMqttClient/src/MqttClient.cpp: In member function 'int MqttClient::unsubscribe(const char*)':
~/Development/Arduino/libraries/ArduinoMqttClient/src/MqttClient.cpp:381:43: warning: variable length array 'packetBuffer' is used [-Wvla]
   uint8_t packetBuffer[5 + remainingLength];
                                           ^
~/Development/Arduino/libraries/ArduinoMqttClient/src/MqttClient.cpp: In member function 'int MqttClient::connect(arduino::IPAddress, const char*, uint16_t)':
~/Development/Arduino/libraries/ArduinoMqttClient/src/MqttClient.cpp:910:43: warning: variable length array 'packetBuffer' is used [-Wvla]
   uint8_t packetBuffer[5 + remainingLength];
                                           ^
~/Development/Arduino/libraries/ArduinoMqttClient/src/MqttClient.cpp: In member function 'int MqttClient::publishHeader(size_t)':
~/Development/Arduino/libraries/ArduinoMqttClient/src/MqttClient.cpp:982:46: warning: variable length array 'packetHeaderBuffer' is used [-Wvla]
   uint8_t packetHeaderBuffer[5 + headerLength];
                                              ^