Add the class actually used in the MusicOnHold start event.

(closes issue #16499)
 Reported by: syspert
 Patches: 
       mohclass.patch uploaded by syspert (license 938)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@238795 65c4cc65-6c06-0410-ace0-fbb531ad65f3