style: Treat PyBind headers as Python headers

style: Treat PyBind headers as Python headers

Some PyBind11 headers need to include Python.h. This means that we need
to include PyBind11 headers before any standard library headers. Enforce
this by applying the Python.hh rules for anything in pybind11/.

Change-Id: Id175a4f613960a17f84f98b81bfd02806e905d5a
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/3120
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Tony Gutierrez <anthony.gutierrez@amd.com>