Moved waveform generation to synth.c from main

generate_wave() takes sr and note_frqs array to generate output wave

I prefer to leave the srate function and note_frqs in main because i
noticed that it happens something strange if i move in other files...
dunno why... if i move, then the program works well with input from
midi keyboard, but stop to play when i send input trough rosegarden,
even if printing on stdeer out[i] shows that data is written
correctly... so, for now, i'll leave them on main source file, with
process() callback.