gitlab CI: add an xserver build stage
Let's build XIT against the current git X server. This will be required once we start using the inputtest driver and no matching server release is available. This is not the nicest approach because we're using an image that's effectively internal to the X server and we just expect it'll work. If the server changes tags we have to keep this here in sync. A better integration is possible (gstreamer does it already) but for now this here will do. Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>