netfilter: ipset: Remove extra whitespaces in ip_set.h

Remove unnecessary whitespaces.

Ported from a patch proposed by Sergey Popovich <popovich_sergei@mail.ua>.

Suggested-by: Sergey Popovich <popovich_sergei@mail.ua>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>