audio/zynaddsubfx: Update to version 3.0.6 .

Changes between 3.0.5 and 3.0.6:

• Enhances watchpoint system for waveform views (GSOC michiboo)
• Add anti-alias mode to add synth oscillator interpolation
• Add 14 bit NRPN support for automations
• Add Tanh, Cubic, and square distortions with additional shaping params
• Add filter parameter smoothing
• Add filter optional LPF to LFO
• Add repeatable envelopes
• Add LFO fade in
• Add moog ladder filter
• Add comb filter
• Add BPM sync
• Add latched key mode
• Re-enable classic MIDI learn
• Increase envelope time parameter resolution
• Fix excessively loud volume bug in Part XML loader
• Fix crash with gcc 10.x.x within file manager
• Fix crash in erroneous patch load
• Fix OSS/etc detection under cygwin
• Fix padsynth memory leak
• Fix weird note-to-note portamento switches
• Reduce fftw resolution from 64bit to 32bit
• Remove Cxxtest dependency
• Fix other bugs

Approved by: pi (implicit)