#245 Building fails on GTK 3 without X11 support

[Chris Mayo]

If GTK is built without X11 support building fails with:

myipc.cpp:28:10: fatal error: gdk/gdkx.h: No such file or directory
   28 | #include <gdk/gdkx.h>
      |          ^~~~~~~~~~~~
compilation terminated.

Attached patch fixes it for me.