gobject: remove -DG_DISABLE_CONST_RETURNS

GObject had a few cases of depending on G_CONST_RETURN not being defined
to 'const'.  Remove those in preparation for deprecation of
G_CONST_RETURN.

gobject/gtype.c

@@ -488,7 +488,7 @@ type_node_any_new_W (TypeNode             *pnode,
   node->global_gdata = NULL;
   
   g_hash_table_insert (static_type_nodes_ht,
-		       g_quark_to_string (node->qname),
+		       (gpointer) g_quark_to_string (node->qname),
 		       (gpointer) type);
   return node;
 }