Move headers to project section to fix archive

Summary:
Follow up to fix some issues with 59407f3. Headers needed to be in the `project` section instead of `private` in xcode so they don't get included in the archive.

cc javache
Closes #11395

Differential Revision: D4313048

Pulled By: javache

fbshipit-source-id: 805dbbe9f149acfe780be76e99c949c450272358

Author: janicduplessis