Fix docs.rs rendering of macos version

There was a problem building libproc-rs macos version in docs.rs, because of the use of bindgen to generate code from macos C headers - as docs.rs uses linux to build and the headers were not available.

I have discussed with docs.rs maintainers and decided to go with a pre-generated set of macos bindings from bindgen, when building in docs.rs. Normal code builds will continue to use bindgen generated bindings as before.

See: https://docs.rs/crate/libproc/latest