xwayland: Use defines for X11 directory and path

Rather than repeating the same strings for X11 directory and path all
around the code, use a define.

No functional change.

(cherry picked from commit 6a4400a1774f3ea782064c9aeb9b07c774d22ee9)

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1788>