res_http_websocket: Export symbol for ast_websocket_set_timeout

Thanks to Sean Bright for pointing out that this was missed in #asterisk-dev.
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Merged revisions 417419 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@417420 f38db490-d61c-443f-a65b-d21fe96a405b