unix: add SizeofNhmsg and SizeofNexthopGrp

These were missed in CL 227357.

For golang/go#38249

Change-Id: Iab75145ae77a027e11bc4cad75300af0a510a537
Reviewed-on: https://go-review.googlesource.com/c/sys/+/716780
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Florian Lehner <lehner.florian86@gmail.com>