Enable use of libpffft in system-side unit tests am: a16a548b69

Original change: https://android-review.googlesource.com/c/platform/external/pffft/+/2281089

Change-Id: I37fd400d21b52d29699d001c19ec60ddeef4595c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>