mapi: remove Xlib/xcb include in gl_marshal.py

mapi: remove Xlib/xcb include in gl_marshal.py

The only use of the header is to provide the _X_INLINE macro. We already
require (and provide where needed) 'inline', plus it's used in the file
already.

So replace the macro and drop the include. This fixes the build on
platforms which lack the header - from X-less Linuxes to Androids.

Fixes: 05dd4a1104e ("glapi: Generate GL API marshalling code from the XML.")
Reported-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100223
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>