Apply PR #4 without Synth_Dexed git submodule

[probonopd]

Credits: @dcoredump

PR #4 refers to Synth_Dexed c6e768820cffd2c9db277ebce3faf67b27efb26b which does not seem to exist. Hence applying the changes this way.

[probonopd]

CPP   main.o
  CPP   kernel.o
  CPP   ../Synth_Dexed/src/synth_dexed.o
../Synth_Dexed/src/synth_dexed.cpp: In static member function 'static void AudioSynthDexed::KeyStatusHandlerRaw(unsigned char, const unsigned char*)':
../Synth_Dexed/src/synth_dexed.cpp:1993:17: error: 'ucKeyNumber' was not declared in this scope; did you mean 'm_ucKeyNumber'?
 1993 |  s_pThis->keyup(ucKeyNumber);
      |                 ^~~~~~~~~~~
      |                 m_ucKeyNumber
make: *** [../circle-stdlib/libs/circle/Rules.mk:[14](https://github.com/probonopd/MiniDexed/runs/5205110565?check_suite_focus=true#step:6:14)3: ../Synth_Dexed/src/synth_dexed.o] Error 1
make: *** Waiting for unfinished jobs....