lib/ffmpeg/Init: fix av_register_all() deprecation warning

av_register_all() was deprecated in
FFmpeg/FFmpeg@0694d87024

src/lib/ffmpeg/Init.cxx

@@ -33,6 +33,9 @@ FfmpegInit()
 {
 	av_log_set_callback(FfmpegLogCallback);
 
+#if LIBAVFORMAT_VERSION_INT < AV_VERSION_INT(58, 9, 100)
+	/* deprecated as of FFmpeg 4.0 */
 	av_register_all();
+#endif
 }