snap: Fixed crash on startup on Wayland

[bjorn]

Such that the Tiled snap will hopefully start working again on Wayland based on XWayland.

Currently we're seeing this when trying to launch Tiled on Wayland:

$ snap run tiled
xkbcommon: ERROR: failed to add default include path /usr/share/X11/xkb
qt.qpa.wayland: failed to create xkb context
QSocketNotifier: Can only be used with threads started with QThread
Segmentation fault (core dumped)

[bjorn]

Removing Wayland support like this yields:

$ snap run tiled
qt.qpa.plugin: Could not find the Qt platform plugin "wayland-egl" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

Aborted (core dumped)