kbuild: use getopt_long(), not its _only() variant

NetBSD lacks getopt_long_only() whereas getopt_long() works just fine.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>