meson-build: libxfce4util-config.h not parsed by Gobject introspection

I had the same problem with libxfce4panel-config.h for the panel and could not solve it. No matter how you pass it to gnome.generate_gir() (including, for example, wrapping configure_file() into custom_target()), it refuses to take it into account. As a result, the generated doc is empty and these symbols are also missing from the vala binding.

This seems to be specific to the fact that it's a header, as I also generate libxfce4panel-enum-types.c (CustomTarget, via gnome.mkenums()) which is taken into account.