It has been suggested to migrate from syscall to golang.org/x/sys
The NewCallback function works in syscall. But the golang.org/x/sys/windows NewCallback function did not work.

Since golang.org/x/sys/windows already imports syscall, the NewCallback function now calls syscall.NewCallback.

Fixes golang/go#21831

Change-Id: I5cabd8540777b398a288308a66ae20896ed4a1db
Reviewed-on: https://go-review.googlesource.com/63250
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>