Fix udp.c compilation with recent glibc (>= 2.8)

Originally committed as revision 14963 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/udp.c

@@ -24,6 +24,7 @@
  * UDP protocol
  */
 
+#define _BSD_SOURCE     /* Needed for using struct ip_mreq with recent glibc */
 #include "avformat.h"
 #include <unistd.h>
 #include "network.h"