issue: Ticket Preview Broken

This addresses an issue where the Ticket Preview is broken no matter if it
has a custom field attached or not. This corrects a typo for `$thisstaff`
so the system can get the Agent’s Role to complete the rest of the preview
options.

include/staff/templates/ticket-preview.tmpl.php

@@ -6,7 +6,7 @@
 
 $staff=$ticket->getStaff();
 $lock=$ticket->getLock();
-$role=$ticket->getRole($thistaff);
+$role=$ticket->getRole($thisstaff);
 $error=$msg=$warn=null;
 $thread = $ticket->getThread();