main: Make gtk_get_option_group() available to bindings

Now that GOptionGroup is registered as a boxed type [1], don't skip it
in gobject-introspection.

[1]: See https://bugzilla.gnome.org/show_bug.cgi?id=743349

https://bugzilla.gnome.org/show_bug.cgi?id=743350

gtk/gtkmain.c

@@ -812,7 +812,7 @@ gtk_set_debug_flags (guint flags)
 }
 
 /**
- * gtk_get_option_group: (skip)
+ * gtk_get_option_group:
  * @open_default_display: whether to open the default display
  *     when parsing the commandline arguments
  *