netfilter: synproxy: remove module dependency on IPv6 SYNPROXY

This is a prerequisite for the infrastructure module NETFILTER_SYNPROXY.
The new module is needed to avoid duplicated code for the SYNPROXY
nftables support.

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>