Add the SizeofIovec constant corresponding to type Iovec.

Change-Id: Icde241b8cac3bc89806f0081a4021598bf0ef7cf
Reviewed-on: https://go-review.googlesource.com/51730Reviewed-by: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>