mrb_include_module support class and module

that not have methods each other

src/class.c

@@ -741,7 +741,7 @@ mrb_include_module(mrb_state *mrb, struct RClass *c, struct RClass *m)
     struct RClass *p = c, *ic;
     int superclass_seen = 0;
 
-    if (c->mt == m->mt) {
+    if (c->mt && c->mt == m->mt) {
       mrb_raise(mrb, E_ARGUMENT_ERROR, "cyclic include detected");
     }
     while (p) {