libavdevice: Define _XOPEN_SOURCE for usleep

This hopefully fixes build failures on Dragonfly BSD.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavdevice/bktr.c

@@ -26,6 +26,7 @@
 
 #define _BSD_SOURCE 1
 #define _NETBSD_SOURCE
+#define _XOPEN_SOURCE 600
 
 #include "libavformat/avformat.h"
 #if HAVE_DEV_BKTR_IOCTL_METEOR_H && HAVE_DEV_BKTR_IOCTL_BT848_H