`gen` is a reserved keyword in Rust 2024 edition

[mdbernard]

Since gen is a reserved keyword in the Rust 2024 edition, it must be added to the KEYWORDS const in sanitize_keywords: https://github.com/rust-embedded/svd2rust/blob/master/src/util.rs#L135-L142, otherwise crates build with Rust 2024 will give compiler errors.