gitlab-ci: add .config to artefacts

The defconfig is dumped at the very beginning of the build log, but the
resulting .config is still interesting to get, in case some settings did
not end up in there (e.g. because of the host gcc version, or because of
conflicting settings, or because the Config.in changed without the tests
being updated, or...).

So, add the .config to the list of artefacts to save.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Acked-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>