Enable use of libpffft in system-side unit tests am: a16a548b69 am: 516d50cef3

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

Change-Id: Ibe9e7cd85a8275ce42421b4ff7ce4a61f33b2d5f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>