use the path from api/

[TingDaoK]

Issue #, if available:

• In case of building s2n taking api/ as the header path, eg: https://github.com/awslabs/aws-crt-swift/blob/main/Package.swift#L100

Description of changes:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.39%. Comparing base (dc41ddc) to head (98959e2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #684   +/-   ##
=======================================
  Coverage   80.39%   80.39%           
=======================================
  Files          28       28           
  Lines        5992     5992           
=======================================
  Hits         4817     4817           
  Misses       1175     1175           

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