rust: pin-init: allow `pub` fields in `derive(Zeroable)`

Add support for parsing `pub`, `pub(crate)` and `pub(super)` to the
derive macro `Zeroable`.

Link: https://github.com/Rust-for-Linux/pin-init/pull/42/commits/e8311e52ca57273e7ed6d099144384971677a0ba


Signed-off-by: Benno Lossin <benno.lossin@proton.me>