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>
3 jobs for wip/xserver-ci-stage in 1 minute and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prep
passed #6153715
container-prep

00:00:12

 
  Install Xserver
passed #6153716
xserver-build

00:01:11

 
  Build
passed #6153717
build

00:00:18