netfilter: nft_compat: set IP6T_F_PROTO flag if protocol is set

ip6tables extensions check for this flag to restrict match/target to a
given protocol. Without this flag set, SYNPROXY6 returns an error.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Patrick McHardy <kaber@trash.net>