Building with INET=n results in

WARNING: "ip_route_output_key" [net/netfilter/nf_conntrack_h323.ko] undefined!

The entire code in net/netfilter is only used for IPv4/IPv6 currently, so
let it depend on INET.

Noticed by Toralf Förster <toralf.foerster@gmx.de>.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>