Build audio.primary.* with BOARD_VNDK_VERSION

Add libhardware_headers to the dependency list since
hardware/libhardware/include is not added to the global include path
when building with BOARD_VNDK_VERSION. Also, a few headers - pthread.h,
unistd.h - are explicitly added.

Bug: 37342627
Test: BOARD_VNDK_VERSION=current m -j audio.primary.msm8998
Change-Id: I908aed7cf9c5aa970d1e4486dcff32d20d44aebd