rust: alloc: add `Box` to prelude

Now that we removed `BoxExt` and the corresponding includes in
prelude.rs, add the new kernel `Box` type instead.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241004154149.93856-15-dakr@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>