lavf: deprecate AVFormatContext.index_built

it's not touched anywhere in ffmpeg, the code setting it was removed
over two years ago (e9b78eeb).
Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>

libavformat/avformat.h

@@ -752,7 +752,9 @@ typedef struct AVFormatContext {
 
     /* av_seek_frame() support */
     int64_t data_offset; /**< offset of the first packet */
-    int index_built;
+#if FF_API_INDEX_BUILT
+    attribute_deprecated int index_built;
+#endif
 
     int mux_rate;
     unsigned int packet_size;

libavformat/version.h

@@ -89,5 +89,8 @@
 #ifndef FF_API_OLD_AVIO
 #define FF_API_OLD_AVIO                (LIBAVFORMAT_VERSION_MAJOR < 53)
 #endif
+#ifndef FF_API_INDEX_BUILT
+#define FF_API_INDEX_BUILT             (LIBAVFORMAT_VERSION_MAJOR < 53)
+#endif
 
 #endif //AVFORMAT_VERSION_H