abstractions/X: allow reading the Xauth file mutter passes to Xwayland.
Bug-Debian: https://bugs.debian.org/935058
Applications running under Xwayland in a GNOME+Wayland session need read access to this file since:
https://gitlab.gnome.org/jwrdegoede/mutter/commit/a8984a81c2e887623d69ec9989ae8a5025f7bd47
… that was first included in mutter 3.33.3.
This rule is presumably only needed for GNOME+Wayland sessions, so one could argue that it should live in abstractions/wayland instead, but Jamie argued that it should be in the X abstraction because Xwayland is a X server.