Add the missing part of r400140

When the patch to add retry-on-forbidden-response was committed, part
of the patch for chan_sip was not committed which caused the feature to
be entirely nonfunctional. This corrects the code in question.

(closes issue ASTERISK-17138)
Review: https://reviewboard.asterisk.org/r/2874
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Merged revisions 405033 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 405081 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 405083 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@405129 65c4cc65-6c06-0410-ace0-fbb531ad65f3