bluetooth: audio: build as static library
bluetooth: audio: build as static library

Make subsys/bluetooth/audio build as a static library.
This is in preparation to enable building the bluetooth
drivers as a static library. That would change the line
"zephyr_include_directories(subsys/bluetooth)" where
this include path is applied to all code compiled.
Its removal will result in subsys/bluetooth/audio not
able to find subsys/bluetooth/common/log.h. So change
subsys/bluetooth/audio to be built as a static library
so it can link to the subsys/bluetooth library where
it can find "log.h" there.

Signed-off-by: Daniel Leung <d94e04d205b5962d7873a139ef298b03c1717f72@intel.com>

galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,galak/zephyr