pjsip: resolve compatibility problem with ast_sip_session

A change in r430179 inserted a variable near the top of a
structure caused a problem when running DPMA in a version
of Asterisk compiled across the change.  This patch moves
the new variable to the end of the structure, eliminating
the problem.

Review: https://reviewboard.asterisk.org/r/4574/
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Merged revisions 433944 from http://svn.asterisk.org/svn/asterisk/branches/13
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Merged revisions 434261 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434263 65c4cc65-6c06-0410-ace0-fbb531ad65f3