variable.c: skip prepended module for constant lookup.
```ruby
module M
FOO = 'm'
end
class A
FOO = 'a'
prepend M
end
class B < A
def foo
p FOO
end
end
B.new.foo  # should print `m` not `a`
```
@@ -775,7 +775,7 @@ const_get(mrb_state *mrb, struct RClass *base, mrb_sym sym)
L_RETRY:
while (c) {
- if (c->iv) {
+ if (!MRB_FLAG_TEST(c, MRB_FL_CLASS_IS_PREPENDED) && c->iv) {
if (iv_get(mrb, c->iv, sym, &v))
return v;
}
