pillowfight.cc: use C linkage for signal handlers

Change-Id: I8447fdf23bbf11d30ca1e49c75a58775f93373b4
Reviewed-on: http://review.couchbase.org/25102
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>