make: remove trailing slashes in trusty-std/rules.mk am: 32c0b35f44

Original change: https://android-review.googlesource.com/c/trusty/lib/+/3403482

Change-Id: I69b448c7f328f9ca12e57be0d23449c6bea7e446
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>