adds friend class YM2612 declaration to line 98

Still throwing errors:
In file included from MegaSynth.h:4,
                 from Trahagean.ino:9:
YM2612.h:456: error: function definition does not declare parameters
YM2612.h:70: error: 'union YM2612::State' is private
YM2612.h:456: error: within this context
YM2612.h:476: error: function definition does not declare parameters
In file included from Trahagean.ino:9:
MegaSynth.h:304: error: function definition does not declare parameters