rust: str: normalize imports in str.rs

Clean up imports in `str.rs`. This makes future code manipulation more
manageable.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250902-rnull-up-v6-16-v7-1-b5212cc89b98@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Andreas Hindborg

rust/kernel/str.rs

@@ -2,12 +2,13 @@
 
 //! String representations.
 
-use crate::alloc::{flags::*, AllocError, KVec};
-use crate::fmt::{self, Write};
+use crate::{
+    alloc::{flags::*, AllocError, KVec},
+    fmt::{self, Write},
+    prelude::*,
+};
 use core::ops::{self, Deref, DerefMut, Index};
 
-use crate::prelude::*;
-
 /// Byte string without UTF-8 validity guarantee.
 #[repr(transparent)]
 pub struct BStr([u8]);