res_pjsip_exten_state: Change 'does not exist' warning to notice

The 'new_subscribe: Extension <> does not exist or has no associated hint'
is a config issue and doesn't need to clutter up logs with warnings.
Changed to notice.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4307/
........

Merged revisions 430319 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@430321 65c4cc65-6c06-0410-ace0-fbb531ad65f3