Revert removal of _NETBSD_SOURCE definition.

It is indeed necessary to compile on NetBSD.

Originally committed as revision 22613 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavdevice/bktr.c

@@ -25,6 +25,7 @@
  */
 
 #define _BSD_SOURCE 1
+#define _NETBSD_SOURCE
 
 #include "libavformat/avformat.h"
 #if HAVE_DEV_BKTR_IOCTL_METEOR_H && HAVE_DEV_BKTR_IOCTL_BT848_H