package/portaudio: Remove threads dependency for alsa module

Portaudio as a whole already depends on threads support, so there is no
need to have an additional dependency for its alsa module.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>