services: gdm: Ensure /var/lib/gdm is owned by "gdm".

Fixes <https://bugs.gnu.org/37423>.
Reported by Jan <tona_kosmicznego_smiecia@interia.pl>.

* gnu/services/xorg.scm (%gdm-activation): New variable.
(gdm-service-type)[extensions]: Add 'activation-service-type'.