poly1305: fix build

Updates golang/go#17422

Change-Id: Ie5f16e24f87b3d800f1182b5b09d6cf638135e33
Reviewed-on: https://go-review.googlesource.com/30970
TryBot-Result: Gobot Gobot <gobot@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
1 file changed