rust: avoid unused import warning in `rusttest`

When compiling for the `rusttest` target, the `core::ptr` import is
unused since its only use happens in the `reserve()` method which is
not compiled in that target:

warning: unused import: `core::ptr`
--> rust/kernel/alloc/vec_ext.rs:7:5
|
7 | use core::ptr;
| ^^^^^^^^^
|
= note: `#[warn(unused_imports)]` on by default

Thus clean it.

Fixes: 97ab3e8eec0c ("rust: alloc: fix dangling pointer in VecExt<T>::reserve()")
Reviewed-by: Alice Ryhl <[e-mail]>
Reviewed-by: Danilo Krummrich <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Miguel Ojeda <[e-mail]>

rust/kernel/alloc/vec_ext.rs

···
 
 use super::{AllocError, Flags};
 use alloc::vec::Vec;
-use core::ptr;
 
 /// Extensions to [`Vec`].
 pub trait VecExt<T>: Sized {
···
         // `krealloc_aligned`. A `Vec<T>`'s `ptr` value is not guaranteed to be NULL and might be
         // dangling after being created with `Vec::new`. Instead, we can rely on `Vec<T>`'s capacity
         // to be zero if no memory has been allocated yet.
-        let ptr = if cap == 0 { ptr::null_mut() } else { old_ptr };
+        let ptr = if cap == 0 {
+            core::ptr::null_mut()
+        } else {
+            old_ptr
+        };
 
         // SAFETY: `ptr` is valid because it's either NULL or comes from a previous call to
         // `krealloc_aligned`. We also verified that the type is not a ZST.