Add _NETBSD_SOURCE definition to fix compilation on NetBSD.

NetBSD is unlikely to fix their headers and the FATE box passes it
as -D_NETBSD_SOURCE on the command line anyway. In this case, it's
better to keep the hack well-contained within this file.
Closes issue 886.
patch by Jorge Acereda, jacereda brainstorm es

Originally committed as revision 22164 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavdevice/bktr.c

@@ -25,6 +25,8 @@
  */
 
 #define _BSD_SOURCE 1
+#define _NETBSD_SOURCE
+
 #include "libavformat/avformat.h"
 #if HAVE_DEV_BKTR_IOCTL_METEOR_H && HAVE_DEV_BKTR_IOCTL_BT848_H
 # include <dev/bktr/ioctl_meteor.h>