Platform-dependent header ignore list for gio docs

This is a bit awkward. A more elegant solution would have
ignored *all* headers and then *un-ignored* some of them
if some conditions were met.

Sadly, we cannot really ignore all headers and then "unignore"
them: that's not how arrays in Meson work.

https://bugzilla.gnome.org/show_bug.cgi?id=794557