usleep is deprecated and disabled in glibc 2.12 unless requested. Use
nanosleep instead if available.

This fixes the following compiler warning:

    ./src/zrtp_iface_scheduler.c: In function 'zrtp_sleep':
    ./src/zrtp_iface_scheduler.c:96:2: warning: implicit declaration of
    function 'usleep' [-Wimplicit-function-declaration]
      usleep(msec*1000);
      ^