Move INET6_ADDRSTRLEN to network.h, similar to other network-related fixups

for broken OSes. This is included in rtsp.h, as opposed to os_support.h.
Should fix OS/2 broken build on fate.

Originally committed as revision 25035 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/os_support.h

@@ -55,10 +55,6 @@ typedef int socklen_t;
 #define closesocket close
 #endif
 
-#ifndef INET6_ADDRSTRLEN
-#define INET6_ADDRSTRLEN INET_ADDRSTRLEN
-#endif
-
 #if CONFIG_FFSERVER
 #if !HAVE_POLL_H
 typedef unsigned long nfds_t;