Add liquid-dsp.

liquid-dsp is a free and open-source digital signal processing (DSP)
library designed specifically for software-defined radios on embedded
platforms. The aim is to provide a lightweight DSP library that does not
rely on a myriad of external dependencies or proprietary and otherwise
cumbersome frameworks. All signal processing elements are designed to be
flexible, scalable, and dynamic, including filters, filter design,
oscillators, modems, synchronizers, complex mathematical operations, and
much more.