xwayland: Make sure /tmp/.X11-unix/ exists

When we're running under a polyinstantiated SELinux environment, we'll
likely start with an isolated and empty /tmp, meannig no /tmp/.X11-unix
directory to add things to. To make it possible to still function in
this kind of setup, make sure said directory exists.

(cherry picked from commit 357f3250)

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