Fix mautrix-signal permissions configuration

Fixup for https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1899

roles/matrix-bridge-mautrix-signal/defaults/main.yml

@@ -99,6 +99,9 @@ matrix_mautrix_signal_relaybot_enabled: false
 matrix_mautrix_signal_bridge_permissions: |
   '*': relay
   '{{ matrix_mautrix_signal_homeserver_domain }}': user
+  {% if matrix_admin %}
+  "{{ matrix_admin }}": admin
+  {% endif %}
 
 # Default configuration template which covers the generic use case.
 # You can customize it by controlling the various variables inside it.