internal/syscall/unix: add hurd build tag

Patch from Svante Signell.

Fixes GCC PR go/93900

Change-Id: I3a86102858d13d16ddec0c646867aed2d4cb6c78
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/220589
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/libgo/go/internal/syscall/unix/nonblocking.go b/libgo/go/internal/syscall/unix/nonblocking.go
index 9b39bb2..1734cdc 100644
--- a/libgo/go/internal/syscall/unix/nonblocking.go
+++ b/libgo/go/internal/syscall/unix/nonblocking.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 dragonfly freebsd linux netbsd openbsd
+// +build dragonfly freebsd hurd linux netbsd openbsd
 
 package unix