inspectrum: add an incomplete formula

[rxseger]

Attempt at adding a formula for https://github.com/miek/inspectrum

Problems:

1. Latest release https://github.com/jgaeddert/liquid-dsp installed by https://github.com/dholm/homebrew-sdr is 1.2.0 from 2012, incompatible with latest inspectrum, compile errors: https://gist.github.com/rxseger/2c0aea05f47e162725f5ec703461d32a (unknown type name 'freqdem'; did you mean 'freqmodem'), unknown how to specify a HEAD dependency, for now manually install:

brew tap dholm/sdr
brew install --HEAD dholm/sdr/liquid-dsp

1. cmake QT5 vs QT5, https://github.com/miek/inspectrum#building-on-osx---homebrew documents the required environment variable for passing to cmake:

CMAKE_PREFIX_PATH=$(brew --prefix qt5)/lib/cmake cmake ..

This works when building inspectrum by hand, but unclear how to pass it to Homebrew properly (this PR is my failed attempt so far what I have tried unsuccessfully)