Migrate to cargo_embargo. am: 71431fc1f7 am: 489cbddeec am: 6ef0bdc16d

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/synstructure/+/2828053

Change-Id: I83520c3e79265d6f6d048531b98b114f0d9f2663
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>