Move AudioOutput back to inside audio processor classes.

Was wasting precious RAM making it static. I lose 400 bytes of .text, not sure why, but I think the trade is worth it.

firmware/baseband/proc_nfm_audio.hpp

@@ -27,6 +27,7 @@
 #include "dsp_decimate.hpp"
 #include "dsp_demodulate.hpp"
 
+#include "audio_output.hpp"
 #include "spectrum_collector.hpp"
 
 class NarrowbandFMAudio : public BasebandProcessor {
@@ -55,6 +56,8 @@ private:
 
 	dsp::demodulate::FM demod;
 
+	AudioOutput audio_output;
+
 	SpectrumCollector channel_spectrum;
 
 	bool configured { false };