Enable introspection of X11-specific functions and methods

Enable introspection of X11-specific functions and methods

Functions dealing with native Xlib types were (skip)ed because
gobject-introspection did not have correct Xlib types declarations.
They are corrected now, so these GdkX11 functions can be enabled back
again.

https://bugzilla.gnome.org/show_bug.cgi?id=655495