Add missing <functional> include am: 5b5fe9ef2d

Original change: https://android-review.googlesource.com/c/platform/external/libldac/+/2759541

Change-Id: I8286e9bdeb9b18639b62f9454da40fea0740061d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>