gitlab-ci.yml: Install libattr-devel and libcap-devel to test virtio-9p

So far the gitlab-ci was not testing virtio-9p yet, since we did not
install libattr-devel and libcap-devel in any of the pipelines. Do
it now to get some more test coverage.

Message-Id: <20190905111729.1197-1-thuth@redhat.com>
Acked-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>