Add CI for real

The stable branch didn't have a gitlab-ci.yml file before so I had to
explicitly git-add it...

.gitlab-ci.yml

+# SPDX-FileCopyrightText: None
+# SPDX-License-Identifier: CC0-1.0
+
+include:
+  - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/android.yml
+  - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/linux.yml
+  - https://invent.kde.org/sysadmin/ci-utilities/raw/master/gitlab-templates/freebsd.yml