host_vars: Add hosts file for gitlab.archlinux.org

Added missing hosts file for gitlab.archlinux.org to make sure the playbook
runs correctly.

host_vars/gitlab.archlinux.org

+---
+filesystem: btrfs
+zabbix_agent_templates:
+  - Template OS Linux
+  - Template App Borg Backup
+  - Template App HTTP Service
+  - Template App HTTPS Service
+  - Template App Nginx
+  - Template App SSH Service