rust: alloc: employ a trailing comment to keep vertical layout

Apply the formatting guidelines introduced in the previous commit to
make the file `rustfmt`-clean again.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc/kvec.rs

@@ -9,7 +9,7 @@
 };
 use crate::{
     fmt,
-    page::AsPageIter,
+    page::AsPageIter, //
 };
 use core::{
     borrow::{Borrow, BorrowMut},