Include os_support.h which has a fallback declaration of socklen_t

This fixes compilation on some OSes

Patch by Dave Yeo, daveryeo at telus dot net

Originally committed as revision 22426 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtsp.c

@@ -30,6 +30,7 @@
 #endif
 #include <strings.h>
 #include "network.h"
+#include "os_support.h"
 #include "rtsp.h"
 
 #include "rtpdec.h"