netfilter: synproxy: add common uapi for SYNPROXY infrastructure

This new UAPI file is going to be used by the xt and nft common SYNPROXY
infrastructure. It is needed to avoid duplicated code.

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