internal/poll: add hurd build tag

Patch from Svante Signell.

Fixes GCC PR go/93900

Change-Id: I40b5d0d13bdb35133c698acab03b1c09097b464e
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/220592
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed