move SkDeque.h into private

Perhaps just make this SkTStack...

Change-Id: Iefdbb1e33acec96aec5f885e3e16ac2d97fd5f73
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/271320
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
8 files changed