rust/drm: import ARef from sync crate

ARef is defined in sync and is getting used from types causing the
build to fail.

Fix this by using ARef from sync module.

Fixes: 80df573af9 ("rust: drm: gem: shmem: Add DRM shmem helper abstraction")
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Link: https://patch.msgid.link/20260426094725.2188668-2-mkchauras@gmail.com
[ Add missing Fixes: tag. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/drm/gem/shmem.rs

@@ -19,10 +19,8 @@ use crate::{
     },
     error::to_result,
     prelude::*,
-    types::{
-        ARef,
-        Opaque, //
-    }, //
+    sync::aref::ARef,
+    types::Opaque, //
 };
 use core::{
     ops::{