Honor ticket number in subject line without brackets

We introduced a glitch in 29b3714, which was the patch that introduced matching on email headers other than the Subject.

I misinterpreted the original `preg_match()` regex, thinking that it was requiring brackets. Looking at it again today, I see that it didn't. The current one does, however, require surrounding brackets.

Fixes osTicket/osTicket-1.8#342