Add alloc dependency for #[no_std] use in the trusty kernel am: 4494fd2edc

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/virtio-drivers/+/2963363

Change-Id: I15f8949ef9b0ec4a196996c060193f7f22317f24
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>