Fix incorrect PyBind11 headers target reference

**Is this pull request associated with an issue(s)?** No. **Description** The current name used to reference the PyBind11 headers target (`pybind11_headers`) only exists if PyBind11 is built with your project. If it is already installed, the exported target is instead `pybind11::headers`. During the build, `pybind11_headers` is aliased to `pybind11::headers` ([here](https://github.com/pybind/pybind11/blob/master/CMakeLists.txt#L279)), so using `pybind11::headers` exclusively should work in both scenarios. **TODOs** - [x] ParallelZone may need to be updated first for this to pass.