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: http://svn.asterisk.org/svn/asterisk/trunk@403378 f38db490-d61c-443f-a65b-d21fe96a405b