main/libao: Update libao to create pulseaudio subpackage

While testing the pianobar package (pandora player for command line) on postmarketOS on my pinephone I discovered that the sound didn't work. After some investigating I found that the pulseaudio extension needed to be enabled for libao, the sound library used by pianobar.

After talking with the folks in the #alpine-devel matrix channel, it seems the right way forward is to have the APKBUILD create a libao-pulse subpackage. This is my first time doing such a thing so please let me know if there are any issues here.

The makedepends on the main package still depends on pulseaudio-dev (not sure if there is a way around that), but only the libpulse.so file is put into the pulse subpackage.

I confirmed that without the pulse subpackage installed, pianobar sound doesn't work for me, and once installed the sound works.