netfilter: arptables: allow xtables-nft only builds

Allows to build kernel that supports the arptables mangle target
via nftables' compat infra but without the arptables get/setsockopt
interface or the old arptables filter interpreter.

IOW, setting IP_NF_ARPFILTER=n will break arptables-legacy, but
arptables-nft will continue to work as long as nftables compat
support is enabled.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Phil Sutter <phil@nwl.cc>