internal/socket: remove unused sizeofMmsghdr const
Change-Id: I6f66e2fd93916438ad959de469ee081b7085fe1a
Reviewed-on: https://go-review.googlesource.com/c/net/+/195597
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>