contrib: bash_completion.d: force zpool symlink recreation

ln will fail if the target already exists, which causes make to bail
out. Adding -f makes it more "compiler-like", overwriting the target
instead.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Mateusz Piotrowski <0mp@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16423