Check non-avutil headers for holes in auto-generated bindings

[FreezyLemon]

ffmpeg-sys already has the avutil subdirectory where some things from libavutil are ported to Rust because bindgen can't find or translate them (e.g. static function implementations in headers, function-like macros etc.).

There are probably things in the other av libraries that are missing in the -sys crate as a result. It's probably a good idea to try to add these, and hide them behind the library's feature flag.