Also add additional packet timestamping constants and types defined
in the same header.

Change-Id: I4280f925b46e229f4c3a83a7fee4612592314016
Reviewed-on: https://go-review.googlesource.com/c/158761
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>