fff-tunneldigger: add missing "/"

Signed-off-by: Robert Langhammer <rlanghammer@web.de>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Jan Kraus <mayosemmel@gmail.com>
Reviewed-by: Steffen Pankratz <kratz00@gmx.de>