package/mpv: simplify alsa-lib feature selection

Instead of having the user need to figure out that 2 alsa-lib features
are needed for mpv ALSA support, have the top-level mpv option select
the relevant alsa-lib features, when alsa-lib is enabled as already done
for pulseaudio in commit 97cd3680

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>