Add dependency for pulseaudio-alsa

Description:

Cosmic requires the pulseaudio-alsa package to be able to change audio volume using media keys (KC_AUDIO_VOL_UP / KC_AUDIO_VOL_DOWN).

Additional info:

• package version(s): 1.0.0.alpha.3-1
• this has been discussed upstream here: https://github.com/pop-os/cosmic-epoch/issues/895
• relates to #1 (closed)