Fix compiler warning in Mac OS X playback example program.
Fix compiler warning in Mac OS X playback example program.

Include <unistd.h> to provide declaration of usleep().

mpruett/audiofile,fabzzap/audiofile,eriser/audiofile,Distrotech/audiofile,Distrotech/audiofile,Distrotech/audiofile,fabzzap/audiofile,mpruett/audiofile,eriser/audiofile,eriser/audiofile,fabzzap/audiofile,mpruett/audiofile