checkpatch: fix incompatibility with old perl

Do not use '/r' modifier which was introduced in perl 5.14.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Fixes: 3e5875afc0f ("checkpatch: check trace-events code style")
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Message-id: 20171004154420.34596-1-vsementsov@virtuozzo.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
