unix: fix build on DragonFly BSD
unix: fix build on DragonFly BSD

The code was broken such that it didn't compile. With this fix, it compiles.

Fixes golang/go#20055.

Change-Id: I9aeb4900f2c99b2ad254be75b4987913cae71e14
Reviewed-on: https://go-review.googlesource.com/41672
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

multiplay/golang-sys,golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,golang/sys,multiplay/golang-sys