Fix: Missing Headers - Linker Errors

High Level Overview of Change

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
[x] Refactor (non-breaking change that only restructures code)
[ ] Tests (you added tests for code that already exists, or your new feature included in this PR)
[ ] Documentation update
[ ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
[ ] Release

[ ] Public API: New feature (new methods and/or new fields)
[ ] Public API: Breaking change (in general, breaking changes should only impact the next api_version)
[ ] libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
[ ] Peer protocol change (must be backward compatible or bump the peer protocol version)