devel/glib20: unbreak GTKDOC=on after 8cef7ff47def

> Checking for pkg-plist issues (check-plist)

> Parsing plist

> Checking for items in STAGEDIR missing from pkg-plist

Error: Orphaned: share/gtk-doc/html/gobject/gtype-non-instantiatable-non-classed.html

> Checking for items in pkg-plist which are not in STAGEDIR

Error: Missing: share/gtk-doc/html/gobject/gtype-non-instantiatable-classed.html

> Error: Plist issues found.

• Error code 1

https://gitlab.gnome.org/GNOME/glib/-/commit/783f1b864008
(cherry picked from commit 66e9d4dd64727c8acae477b64708d24112a7085c)