pillow: drop the pybind11 build dependency

pillow: drop the pybind11 build dependency

Pillow uses pybind11 only for pybind11.setup_helpers.ParallelCompile,
which parallelises the C extension build; it ships no runtime bindings.
Patch setup.py to remove that import and call and drop pybind11 from
build-system.requires, then remove python-pybind11/host from
PKG_BUILD_DEPENDS. The C extensions are compiled serially instead, with
no functional change to the package.

Signed-off-by: Alexandru Ardelean <redacted>