src/onset/onset.c: add logarithmic compression

src/onset/onset.c: add logarithmic compression

Using these default parameters with the good old HFC, we reach the
following scores:

mirex-2005 | F  80.393 | P  80.841 | R  79.950 | TP 7616 | FP 1360 | Tot 9526
holzapfel  | F  69.441 | P  77.803 | R  62.702 | TP 1360 | FP  388 | Tot 2169