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>
1 file changed