fix build with immutable src

[niklasf]

The source directory is generally assumed to be immutable. Place generated files into OUT_DIR instead.

• OUT_DIR: https://doc.rust-lang.org/cargo/reference/environment-variables.html
• Bindgen suggesting to use it: https://rust-lang.github.io/rust-bindgen/tutorial-4.html
• docs.rs build failing: https://docs.rs/crate/ipset/0.7.1/builds/1213512

[lazytiger]

I put binding.rs in the src folder for IDE code assistant.

[niklasf]

Mhh ... I don't have a great solution :(

All things considered, giving this up and generating the file in OUT_DIR is probably best. rust-analyser would still be able to handle it, text-based autocomplete not so much.

Alternatively, going back to manually generating the file in src is not great, but still better than failing to build with read-only src.