puppet: Add missing dependency on postgresql-common.

postgresql-$(version) depends on postgresql-common depends on ssl-cert,
which creates the ssl-cert group.

puppet/zulip/manifests/postgres_common.pp

@@ -52,6 +52,7 @@ class zulip::postgres_common {
   @user { 'postgres':
     groups     => ['ssl-cert'],
     membership => minimum,
+    require    => Package["postgresql-${zulip::base::postgres_version}"],
   }
   User <| title == postgres |> { groups +> "zulip" }
 }