skhdr::AdaptiveGlobalToneMap: Add SK_API to sub-structures

Also remove operator== because the body doesn't exist.

Bug: 468928417
Change-Id: Ie05548f5047e90e7169c9e224c0df9d7251c662f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1140556
Commit-Queue: Christopher Cameron <ccameron@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Reviewed-by: Christopher Cameron <ccameron@google.com>