Add _XOPEN_SOURCE definition for usleep().

patch by Dave Yeo, daveryeo telus net

Originally committed as revision 24893 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffplay.c

@@ -19,6 +19,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#define _XOPEN_SOURCE 600
+
 #include "config.h"
 #include <inttypes.h>
 #include <math.h>