rust: pin-init: Implement `Wrapper` for `UnsafePinned` behind feature flag.

Add the `unsafe-pinned` feature which gates the `Wrapper`
implementation of the `core::pin::UnsafePinned` struct.

For now this is just a cargo feature, but once `core::pin::UnsafePinned`
is stable a config flag can be added to allow the usage of this
implementation in the linux kernel.

Signed-off-by: Christian Schrefl <chrisi.schrefl@gmail.com>
Link: https://github.com/Rust-for-Linux/pin-init/pull/37/commits/99cb1934425357e780ea5b0628f66633123847b8


[ Fixed commit authorship. - Benno ]
Signed-off-by: Benno Lossin <benno.lossin@proton.me>