Ensure gnupg installed on Raspbian

It's likely installed by default, but it doesn't hurt to specify it.
It also makes us more the same with `setup_debian.yml`.

roles/matrix-base/tasks/server_base/setup_raspbian.yml

@@ -5,6 +5,7 @@
     name:
       - apt-transport-https
       - ca-certificates
+      - gnupg
     state: present
     update_cache: yes