Do not export to the public the first_protocol symbol at the next

major bump.
There is no need for that, since av_protocol_next() already provides
access to the first registered protocol.

Originally committed as revision 17326 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avio.h

@@ -137,7 +137,10 @@ typedef struct URLProtocol {
                              int64_t timestamp, int flags);
 } URLProtocol;
 
+#if LIBAVFORMAT_VERSION_MAJOR < 53
 extern URLProtocol *first_protocol;
+#endif
+
 extern URLInterruptCB *url_interrupt_cb;
 
 /**