stasis: set a channel variable on websocket disconnect error

Resolve compile errors caused by r433839 by included the missing
header file, pbx.h.
........

Merged revisions 433863 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@433868 65c4cc65-6c06-0410-ace0-fbb531ad65f3