Fix generation of Trusty makefile rules am: af191c320d

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/der/+/2904248

Change-Id: Id00d7771cc6417daa29fdb75129aa35108b4676c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>