weston-launch: always run through all groups

If the user is in group 0, we'd exit the loop early with a failure. Make sure
we run through all groups.

https://gitlab.freedesktop.org/wayland/weston/issues/86



Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
[Pekka: fix one whitespace]
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>