`-Wvla` compiler warnings

```
~/Development/Arduino/hardware/test/mbed_opta/libraries/SocketWrapper/src/MbedUdp.cpp: In member function 'virtual int arduino::MbedUDP::endPacket()':
~/Development/Arduino/hardware/test/mbed_opta/libraries/SocketWrapper/src/MbedUdp.cpp:76:22: warning: variable length array 'buffer' is used [-Wvla]
   uint8_t buffer[size];
                      ^
```