Expose the chan_pjsip implementation pvt and session in a defined manner.

This allows modules outside of chan_pjsip itself to get the session given
only an Asterisk channel.

Review: https://reviewboard.asterisk.org/r/2674/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@395102 f38db490-d61c-443f-a65b-d21fe96a405b
Add missing newline

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@395107 f38db490-d61c-443f-a65b-d21fe96a405b