Define _XOPEN_SOURCE as 600 before including a XSI extension header.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20871 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavdevice/x11grab.c

@@ -35,6 +35,8 @@
  * and Edouard Gomez <ed.gomez@free.fr>.
  */
 
+#define _XOPEN_SOURCE 600
+
 #include "config.h"
 #include "libavformat/avformat.h"
 #include <time.h>