netfilter: nft_payload: move struct nft_payload_set definition where it belongs

[ Upstream commit ac1f8c04 ]

Not required to expose this header in nf_tables_core.h, move it to where
it is used, ie. nft_payload.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: 33c563eb ("netfilter: nft_payload: skbuff vlan metadata mangle support")
Signed-off-by: Sasha Levin <sashal@kernel.org>