[proaudio] New ebuild: postfish

[Jannis Achstetter <kripton@xxxxxxxxxxxxxx>]

Postfish (from xiph.org) is a digital audio post-processing,
restoration, filtering and mixdown tool.

I used it to declip a recorded audio file.

Ebuild is bit of a hack but works like a charm for me:
https://bugs.gentoo.org/show_bug.cgi?id=439396

Feel free to add to overlay