.gitlab-ci.yml: Increase resources for samba-ad-dc-backup job

This job seems to need more resources, possibly after
b6b5b5fe355fee2a4096e9214831cb88c7a2a4c6 and
565341baf537c9b3145a882d6a23ad4f155c0036

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Fri Jun 28 18:48:49 UTC 2019 on sn-devel-184
