netfilter: cthelper: add missing attribute validation for cthelper

Add missing attribute validation for cthelper
to the netlink policy.

Fixes: 12f7a505

 ("netfilter: add user-space connection tracking helper infrastructure")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>