minor compile issue in mopo/src/wave.h

[ColumPaget]

Hi,

cursynth failed to make on my system with the error:

In file included from mono_panner.cpp:20:0:
wave.h: In static member function 'static mopo::mopo_float mopo::Wave::whitenoise()':
wave.h:245:28: error: 'rand' was not declared in this scope
return (2.0 * rand()) / RAND_MAX - 1;
^
wave.h:245:33: error: 'RAND_MAX' was not declared in this scope
return (2.0 * rand()) / RAND_MAX - 1;

Adding the header <stdlib.h> to mopo/src/wave.h fixed this