Move SkChecksum.h from include/private to src/core

Change-Id: I455f4e64462eb4d96f17bee348b8fddcae2d96cf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/698838
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
23 files changed