rust: io: move module entry to its correct location

The module entry of `io` falsely ended up in the "use" block instead of
the "mod" block, hence move it to its correct location.

Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/lib.rs

@@ -51,6 +51,7 @@ pub mod error;
 pub mod firmware;
 pub mod fs;
 pub mod init;
+pub mod io;
 pub mod ioctl;
 pub mod jump_label;
 #[cfg(CONFIG_KUNIT)]
@@ -87,7 +88,6 @@ pub mod workqueue;
 
 #[doc(hidden)]
 pub use bindings;
-pub mod io;
 pub use macros;
 pub use uapi;