internal/syscall/windows: unexport Version

internal/syscall/windows: unexport Version

windows.Version is just a thin wrapper around RtlGetNtVersionNumbers,
which is an undocumented Windows API.

This CL unexports windows.Version so it is harder to use by accident.

Change-Id: Ib782da04e4e8be66970111a75f5c2df27ef51643
Reviewed-on: https://go-review.googlesource.com/c/go/+/570055
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>