gh-134455: Fix `build-details.json` to use the ``c_api.headers`` key (#134456)

Fix `build-details.json` generation to use the correct `c_api.headers`
key as defined in PEP 739, instead of `c_api.include`.

Co-authored-by: Filipe Laíns 🇵🇸 <lains@riseup.net>