Add support for gqrx-digital fork of gqrx

This pull adds recipes required for building gqrx-digital fork, which contains DSD integration in GQRX. Available at https://github.com/argilo/gqrx_fork/tree/digital

Includes recipes for dependency packages: gr-dsd, libosmo, libshout, libsndfile, mbelib, osmo-tetra

Just want to put this out there in case anybody else is looking for something similar... It's a pretty cool fork of gqrx that helps avoid a lot of manual work, chaining together audio monitors, etc.