Make syn available to product and vendor am: 4b98a0b558 am: a29266e70b am: 4b3069129f

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/syn/+/2476463

Change-Id: I915db50e3d043c70456209f63a0d7c47fd624378
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>