unix: add missing NFT_PAYLOAD_* consts on linux

Change-Id: Id222e11029596d2ea2679e065780171ed3c7708a
GitHub-Last-Rev: 649d64b1c3caf529150cc4ea14061f26b7625e3c
GitHub-Pull-Request: golang/sys#250
Reviewed-on: https://go-review.googlesource.com/c/sys/+/681435
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>