Fix encoding of X11GetKeyboardMappingRequest

[GZGavinZhao]

Minimum repro:

import 'package:x11/x11.dart';

void main() async {
  var client = X11Client();
  await client.connect();

  final mapping = await client.getKeyboardMapping(firstKeycode: 100, count: 1);
  print(mapping);

  await client.close();
}

Correct encoding of X11GetKeyboardMappingRequest borrowed from python-xlib here.

[robert-ancell]

Thanks!