meson: Bump required avutil version

This is needed after 95afdcd .
See 57c16323f26f6d726edb2fd7f83836187f058ed7 in ffmpeg for the relevant
version bump.

meson.build

@@ -56,7 +56,7 @@ endif
 deps = [
     dependency('libavcodec', version: '>=60.31.0'),
     dependency('libavformat', version: '>=60.16.0'),
-    dependency('libavutil', version: '>=58.29.0'),
+    dependency('libavutil', version: '>=58.31.0'),
     dependency('libxxhash'),
 ]