netfilter: fix IP_VS dependencies

When NF_CONNTRACK is enabled, IP_VS uses conntrack symbols.
Therefore IP_VS can't be linked statically when conntrack
is built modular.

Reported-by: Justin P. Mattock <justinmattock@gmail.com>
Tested-by: Justin P. Mattock <justinmattock@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>