unix: add missing nft conntrack constants

The following constants were missing for nft conntrack:

- NFT_CT_SRC_IP
- NFT_CT_DST_IP
- NFT_CT_SRC_IP6
- NFT_CT_DST_IP6
- NFT_CT_ID

https://git.netfilter.org/libnftnl/tree/include/linux/netfilter/nf_tables.h?id=7a7722ee336fac14690851f6c84965582a741601#n1173

Change-Id: I2f9bd12c1ab8569ee019310c040b46b214f65b76
GitHub-Last-Rev: 8fac63b1877a293a03576db26a4aae3121c80d3a
GitHub-Pull-Request: golang/sys#257
Reviewed-on: https://go-review.googlesource.com/c/sys/+/689922
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <mark@golang.org>