internal/syscall/windows: use unsafe.Slice instead of unsafeheader package

internal/syscall/windows: use unsafe.Slice instead of unsafeheader package

Follow CL 428777.

Change-Id: I5ce49322e92c5d6539bb08248e3366187c30dcd8
Reviewed-on: https://go-review.googlesource.com/c/go/+/428780
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>