rust: Add SPDX headers to alloc::vec::{spec_extend, set_len_on_drop}

Add the missing SPDX headers to these modules, which were just imported
from the Rust stdlib. Doing this in a separate commit makes it easier to
audit that the files have not been modified in the original import.

See the preceding two commits for attribution and licensing details.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Link: https://lore.kernel.org/r/20230224-rust-vec-v1-3-733b5b5a57c5@asahilina.net


[ Reworded for typo. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>