Add gitlab CI

A very simple CI for now, just make sure it builds.

Largely inspired by the implementation in libinput by Peter Hutterer
<peter.hutterer@who-t.net>.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
5 jobs for gitlab-ci in 56 seconds (queued for 4 seconds)