Use unsafe extern

[yutannihilation]

rust-lang/rust#127921 says:

Starting in Rust 2024, all extern blocks must be marked as unsafe.

So, in order to be compatible with the 2024 edition, all extern blocks must be marked as unsafe explicitly. However, this feature is stabilized at Rust 1.82. So, this requires a version switch by rustversion.

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