MAINTAINERS: rust: add tree field for RUST [ALLOC]

In the Rust subsystem we are starting to add new subentries which will
have their own trees. Those trees will be part of linux-next and will
be sent as PRs to be merged into rust-next.

Thus do the same for the existing subentry we already have: RUST [ALLOC].

Acked-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250308164258.811040-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>