Updated FilteringAudioSource.cpp to support JUCE IIRFilter refactor

JUCE commit at https://github.com/julianstorer/JUCE/commit/f5ede30892f7f542b68737f9572d423b5ff06df6 broke dRowAudio's calls to IIRfilter. PitchDetector.cpp requires a similar change, which I will send as a separate pull request. BiquadFilter.cpp still needs fixed as well, although I'm still figuring out how to do that.