Adds missing argument for Ticket::lookup for ajax.

Couldn't find the rebase to this.. actually, I didn't look, I'm still struggling with git and eclipse, would probably have been easier with vim! Anyway, rewrote changes for 1.7.
Update ajax.tickets.php

Missed one.. :-(
Merge pull request #878 from greezybacon/issue/ticket-lock-lookup

Fix parameter count to TicketLock::lookup

Reviewed-By: Peter Rotich <peter@osticket.com>