Fix compile issue on void* (#478)

* Fix compile issue on void*

compile issue on Epson AT-991, Clear Linux OS Version 1, Intel icc 2021.1 Beta 20200827 and icpc 2021.1 Beta 20200827 for intel 64

Azure/azure-iot-sdk-c#1755

* Update socketio_berkeley.c

adapters/socketio_berkeley.c

@@ -946,7 +946,7 @@ int socketio_send(CONCRETE_IO_HANDLE socket_io, const void* buffer, size_t size,
                         /* queue data */
                         size_t bytes_sent = (send_result < 0 ? 0 : send_result);
 
-                        if (add_pending_io(socket_io_instance, buffer + bytes_sent, size - bytes_sent, on_send_complete, callback_context) != 0)
+                        if (add_pending_io(socket_io_instance, (const unsigned char*)buffer + bytes_sent, size - bytes_sent, on_send_complete, callback_context) != 0)
                         {
                             LogError("Failure: add_pending_io failed.");
                             result = MU_FAILURE;