network: Define ENOTCONN as WSAENOTCONN if not defined

This fixes compilation with old mingw.org toolchains, which has got
much fewer errno.h entries.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/network.h

@@ -50,6 +50,9 @@
 #ifndef EINPROGRESS
 #define EINPROGRESS     WSAEINPROGRESS
 #endif
+#ifndef ENOTCONN
+#define ENOTCONN        WSAENOTCONN
+#endif
 
 #define getsockopt(a, b, c, d, e) getsockopt(a, b, c, (char*) d, e)
 #define setsockopt(a, b, c, d, e) setsockopt(a, b, c, (const char*) d, e)