stagefright: Set bit_width if ENABLE_AV_ENHANCEMENTS is defined

the compiler failed with error "has no member named bit_width" because bit_width is set only ifdef ENABLE_AV_ENHANCEMENTS

https://github.com/CyanogenMod/android_frameworks_av/blob/cm-11.0/media/libstagefright/AudioPlayer.cpp#L228

Change-Id: I931879f09164482fc09e53a4135d88537b05c7ff