audio: legacy: Add directtrack cflag

Change-Id: I72437dc6a05937c04d07734eabca9d5eebce228e
hal: Compilation fixes

 * Properly include path to kernel headers for postproc effects
 * Don't try to build SSR or Listen if QC_PROP_ROOT isn't set

Change-Id: I8f2a874bbbec09cea8eae423a82e6cc59d03f422