audio: uapi: create audio_kernel_headers target

Create a audio_kernel_headers target so that all audio UAPI
header files can be copied to the right target specific OUT
directory and make it easy for clients to mark dependency on
the header files.

Change-Id: I09ca422cbdd067e6e56fff13deaa4e92de3de1fc
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>