net: add hurd build tag

Patch from Svante Signell.

Change-Id: I8b4efdc7a6c92ce74b9544b7fb8d0c72d74f5600
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/257857
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>