Use `unsafe extern` block for Rust 2024

[yutannihilation]

This is a bit too early to worry about, but it seems Rust 2024 is going to require an explicit unsafe on every extern block.

https://doc.rust-lang.org/nightly/edition-guide/rust-2024/unsafe-extern.html

However, unsafe extern is a very new syntax, which will be available only after Rust 1.82. So, it would be a headache to support both the 2021 edition and the 2024 edition (I'll probably use rustversion to switch the implementations).

You can follow the discussion on bindgen's side here:

rust-lang/rust-bindgen#2901