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/certified/branches/13.1@433864 65c4cc65-6c06-0410-ace0-fbb531ad65f3

apps/app_stasis.c

@@ -34,6 +34,7 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 
 #include "asterisk/app.h"
 #include "asterisk/module.h"
+#include "asterisk/pbx.h"
 #include "asterisk/stasis.h"
 #include "asterisk/stasis_app_impl.h"