Include mathematics.h for av_rescale().
https://bugs.gentoo.org/384281

--- a/src/mediadec_ffmpeg.cpp
+++ b/src/mediadec_ffmpeg.cpp
@@ -29,6 +29,7 @@ extern "C" {
 #include <swscale.h>
 #else
 #include <libavformat/avformat.h>
+#include <libavutil/mathematics.h>
 #include <libswscale/swscale.h>
 #endif
 }