shared/xcb-xwayland: Add missing atoms

Particularly important was _XWAYLAND_ALLOW_COMMITS atom which caused
some annoying flicker when resizing or hoovering over buttons.

This was introduced with 'shared/xcb-xwayland: Split into common
helpers' and somehow I missed those atoms.

Fixes 49d65322

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>