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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@405033 f38db490-d61c-443f-a65b-d21fe96a405b