buildroot

gnuradio: make gr-audio select alsa-lib if no backend selected

gr-audio may be built with alsa-lib or portaudio support. To simplify
dependencies if no backend is selected, we automatically select alsa-lib
as the default audio backend if portaudio is not enabled.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>