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>
diff --git a/libgo/go/net/fd_posix.go b/libgo/go/net/fd_posix.go
index 53abd15..b2f99bc 100644
--- a/libgo/go/net/fd_posix.go
+++ b/libgo/go/net/fd_posix.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris windows
+// +build aix darwin dragonfly freebsd hurd linux netbsd openbsd solaris windows
 
 package net