Build librespot with PulseAudio support in addition to ALSA

This PR compiles librespot with support for PulseAudio, in addition to the ALSA backend.

Projects like balena-sound use librespot and Alpine Linux, but need to compile librespot from scratch to use PulseAudio.

Including the pulse audio backend removes the need for PulseAudio Alpine users to compile their own version of the library.

This is my first pull request for Alpine, so please let me know what I should do for testing etc.