MAINTAINERS: add entry for the `pin-init` crate

Add maintainers entry for the `pin-init` crate.

This crate is already being maintained by me, but until now there
existed two different versions: the version inside of the kernel tree
and a user-space version at [1]. The previous patches synchronized these
two versions to reduce the maintenance burden. In order to keep them
synchronized from now on, separate the maintenance from other Rust code.

Link: https://github.com/Rust-for-Linux/pin-init

 [1]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
Link: https://lore.kernel.org/r/20250308110339.2997091-23-benno.lossin@proton.me


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>