Make res_http_websocket an optional dependency on supported platforms for chan_sip.

(closes issue ASTERISK-20439)
Reported by: sruffell
Patches:
     0001-chan_sip-websocket-support-is-an-optional-API.patch uploaded by sruffell (license 5417)
........

Merged revisions 373914 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373915 65c4cc65-6c06-0410-ace0-fbb531ad65f3