Make synstructure available to product and vendor am: ce999b0fc0 am: 53783e2b57 am: b39a0f6050 am: 776ec4b0a2

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/synstructure/+/2475390

Change-Id: If7fbb6f6697fd22d4ecb50871c96b54c1c337719
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>