lavf: move urlcontext_child_class_next() to protocols.c

It needs to access the list of protocols directly, so it more properly
belongs there.

libavformat/url.h

@@ -264,6 +264,8 @@ int ff_url_join(char *str, int size, const char *proto,
 void ff_make_absolute_url(char *buf, int size, const char *base,
                           const char *rel);
 
+const AVClass *ff_urlcontext_child_class_next(const AVClass *prev);
+
 extern const URLProtocol *ff_url_protocols[];
 
 #endif /* AVFORMAT_URL_H */