pillowfight.cc: use SIG_IGN instead

Change-Id: I733cc262b3d3be53881a973f6dc88e4240cae0aa
Reviewed-on: http://review.couchbase.org/25106
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>