Fixes build with only `-Dgtk_doc=true` without
`-Dintrospection=enabled`:
Program gi-docgen found: NO
Configuring gdk4.toml using configuration
docs/reference/gdk/meson.build:13:2: ERROR: Tried to use not-found external program in "command"
`introspection` is `auto` by default.
The rest of the docs build is only contingent on
`if get_option('gtk_doc')` so we should use the same restriction here.
We check for `build_gir` below already, and the gi-docgen subproject
itself does not need gobject-introspection so it's fine to do it like
this.
4346a33a63
