unix: fix //sys decl after CL 548795

CL 548795 mistakenly changed one of the //sys declaration to "// sys"
while resolving a merge conflict.

For golang/go#64710

Change-Id: I029b461fd3ca7b76f3bd23b3343f7e63c4bb09f3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/688857
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>