res_pjsip_registrar: undefined function pointer symbol

Used a static wrapper around the offending function to alleviate the issue.

Reported by: rmudgett
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Merged revisions 403377 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@403378 65c4cc65-6c06-0410-ace0-fbb531ad65f3