netfilter.mk: fix redirect module locations for 3.19+

ntf_redir_ipvX is part of NAT support, so they should be in the
appropriate nft-nat kmods. Since they depend on nf_nat_redirect,
nf_nat_redirect should be part of nf-nat, not ipt-nat.

Fixes nft-core gaining a missing dependency on nf_nat_redirect.ko.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46109 3c298f89-4303-0410-b956-a3cf2f4a3e73