applehttp: define _XOPEN_SOURCE=600 as required for usleep()

Originally committed as revision 24839 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/applehttp.c

@@ -25,6 +25,7 @@
  * http://tools.ietf.org/html/draft-pantos-http-live-streaming
  */
 
+#define _XOPEN_SOURCE 600
 #include "libavutil/avstring.h"
 #include "avformat.h"
 #include "internal.h"