x/sys/unix is vendored into the standard library, and the commit hook
for the standard library requires files to be gofmt-clean.

Updates golang/go#26924

Change-Id: I22a994062bcdbebe8a1fe1ae0ed4606837f03079
Reviewed-on: https://go-review.googlesource.com/c/162990Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>