netfilter: fix dependencies of kmod-ipt-nat6

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42601 3c298f89-4303-0410-b956-a3cf2f4a3e73
netfilter: some more dependency hell fixes

Signed-off-by: Steven Barth <steven@midlink.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42602 3c298f89-4303-0410-b956-a3cf2f4a3e73