New FFmpeg public API headers to match our source tarball in deps.

third_party/ffmpeg/include/libavutil/fifo.h

Index: third_party/ffmpeg/include/libavutil/fifo.h
diff --git a/third_party/ffmpeg/include/libavutil/fifo.h b/third_party/ffmpeg/include/libavutil/fifo.h
index d3538447159f03462e3a311481671f2a7b3531e0..a904dfd02eec40b832b06dffc1c7923f2dddeaa2 100644
--- a/third_party/ffmpeg/include/libavutil/fifo.h
+++ b/third_party/ffmpeg/include/libavutil/fifo.h
@@ -62,6 +62,14 @@ void av_fifo_reset(AVFifoBuffer *f);
 int av_fifo_size(AVFifoBuffer *f);
 
 /**
+ * Returns the amount of space in bytes in the AVFifoBuffer, that is the
+ * amount of data you can write into it.
+ * @param *f AVFifoBuffer to write into
+ * @return size
+ */
+int av_fifo_space(AVFifoBuffer *f);
+
+/**
  * Feeds data from an AVFifoBuffer to a user-supplied callback.
  * @param *f AVFifoBuffer to read from
  * @param buf_size number of bytes to read