meson: Require gi-docgen when gtk_doc is required

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.

docs/reference/meson.build

@@ -1,9 +1,7 @@
 toml_conf = configuration_data()
 toml_conf.set('version', meson.project_version())
 
-gidocgen = find_program('gi-docgen',
-                        required: get_option('gtk_doc') and get_option('introspection').enabled(),
-                        native: true)
+gidocgen = find_program('gi-docgen', required: get_option('gtk_doc'), native: true)
 
 gidocgen_common_args = [
   '--quiet',