Add Trusty rules.mk file am: 8453e54e2e

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/downcast-rs/+/2770001

Change-Id: I00339cac2ca531ca634b73722c8ee73febb91367
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>