chan_misdn: Fix compile error.

(issue ASTERISK-15456)
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Merged revisions 379226 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 379230 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@379231 65c4cc65-6c06-0410-ace0-fbb531ad65f3