bug#53914: Mesa and xkbcomp are hardcoded in xorg-wrapper

[Gordon Quad via Bug reports for GNU Guix]

mesa and xkbcomp packages are not included in xorg-server-configuration, cannot 
be customized and are hardcoded here:

https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/services/xorg.scm#n356