roles/synapse: Add cert role dependency

The *cert* role must be defined as a role dependency now, so that the
role can define a handler to "listen" for the "certificate changed"
event.  This change happened on *master*, before the *matrix* branch was
merged.

roles/synapse/handlers/main.yml

@@ -1,4 +1,5 @@
 - name: restart synapse
+  listen: certificate changed
   service:
     name: synapse
     state: restarted