libstagefright: ifdef the audio_defs.h include for QCOM_DIRECTTRACK

Devices that use BOARD_USES_LEGACY_ALSA_AUDIO do not use this file,
this was breaking build for all 8960 devices.

Change-Id: Ic0ae606615f7ccc72cef9836d52bfa7045f326ae
Merge pull request #5 from CyanogenMod/cm-12.0

libstagefright: ifdef the audio_defs.h include for QCOM_DIRECTTRACK