Use srtp_shutdown when available

This allows the SRTP library to be shut down properly when the
functionality is offered by libsrtp.

Review: https://reviewboard.asterisk.org/r/2538/
(closes issue ASTERISK-21719)
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Merged revisions 388768 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 388769 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@388770 65c4cc65-6c06-0410-ace0-fbb531ad65f3