meson: Allow building on a Wayland-only environment

Currently, it is not possible to build spice-gtk on a Wayland-only
environment due to the x11 dependency in meson.

Since commit a7381b08 ("Allow to build without X11 gdk backend)
it is possible to build spice-gtk without X11 GDK backend, so make
the x11 dependency in meson optional.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Frediano Ziglio <freddy77@gmail.com>