To keep things sane, move SkNx and SkHalf to include/private
Bug: skia:
Change-Id: Iaee8c3f92a2720bf97583c3f326486d4ce462e8b
Reviewed-on: https://skia-review.googlesource.com/c/165344
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
