Don't install pybind11 header directory for system pybind11 installs (#30758)
Summary:
For system pybind11 installs this is a system header location that should not get installed since it might include other unrelated headers. Since the header is already installed for a system install there's no need to install the headers, so only do the install when we use the bundled pybind11 version.
Closes https://github.com/pytorch/pytorch/issues/29823. Closes https://github.com/pytorch/pytorch/issues/30627.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30758
Differential Revision: D18820189
Pulled By: bddppq
fbshipit-source-id: fcc9fa657897e18c07da090752c912e3be513b17
