gitlab CI: add a basic build test

Uses FreeBSD 13.0 for build environment since this driver is not supported on Linux and FreeBSD is the platform the driver supports that has a template available under https://gitlab.freedesktop.org/freedesktop/ci-templates/-/tree/master/templates.

This was particularly fun, since attempting to install the xorg-server package for FreeBSD filled the disk in the qemu image after pulling in all of Mesa, LLVM, etc. So instead, we clone the xserver git and build it without GLX/Glamor support, since those don't affect input drivers.

Signed-off-by: Alan Coopersmith alan.coopersmith@oracle.com