internal/syscall/unix: add hurd build tag

Patch from Svante Signell.

Fixes GCC PR go/93900

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/220589

gcc/go/gofrontend/MERGE

-d5d00d310ec33aeb18f33f807956ec0c4eeea6bb
+4e6402f3153fad368830ff3fe57c6302179de806
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

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