@[deleted]: as Corey pointed out you can already do that from the select inside the ticket view, however I understand that sometimes you just want to click once instead of twice right? ;)
So you can edit the file include/staff/viewticket.inc.php and place the following code (line 120 approx. before the table):
&a=process&do=close&ticket_id=<?=$id?>">Close this ticket
That should the work for you, tell me how it goes