rust: block: rnull: Adapt to asahi's rust base branch

Signed-off-by: Janne Grunau <j@jannau.net>

Author: jannau

drivers/block/rnull.rs

@@ -20,6 +20,7 @@ use kernel::{
     error::Result,
     new_mutex, pr_info,
     prelude::*,
+    str::CStr,
     sync::{Arc, Mutex},
     types::ARef,
 };