netfilter: x_tables: fix build with CONFIG_COMPAT=n

I placed the helpers within CONFIG_COMPAT section, move them
outside.

Fixes: 472ebdcd ("netfilter: x_tables: check error target size too")
Fixes: 07a9da51 ("netfilter: x_tables: check standard verdicts in core")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>