Since extending a class means including on the singleton class of the
class, this should now complain this:

``` ruby
module M
  extend Gitlab::Utils::Override

  override :f
  def f
    super.succ
  end
end

class C
  extend M

  def self.f
    0
  end
end
```

It should complain because `C.f` wasn't calling `M#f`.
This should pass verification:

``` ruby
module M
  extend Gitlab::Utils::Override

  override :f
  def f
    super.succ
  end
end

class B
  def self.f
    0
  end
end

class C < B
  extend M
end
```

Because `C.f` would now call `M#f`, and `M#f` does override something.