API documentation fails to build with gtk-doc 1.30

I know that master has moved away from gtk-doc but the latest release fails with gtk-doc 1.30:

gst-libav-plugins-docs.sgml:38: element include: XInclude error : could not load xml/tree_index.sgml, and no fallback was found

It's not wrong: that file wasn't generated. object_index.sgml exists, but this doesn't.