include systemd class in a timer dropin

manifests/timer/dropin.pp

@@ -65,6 +65,8 @@ define systemd::timer::dropin (
   #     "Not a supported documentation uri: ${documentation} - It has to be one of 'http://', 'https://', 'file:', 'info:' or 'man:'")
   # }
 
+  contain systemd
+
   if ($persistent != undef and $persistent == true and $on_calendar == undef)
   {
     fail('$persistent being "true" only works with $on_calendar being set.')