Byte compilation warning "the function ‘js2-imenu-extras-setup’ might not be defined at runtime."

This simple code:

```emacs-lisp
(use-package js2-imenu-extras
  :functions js2-imenu-extras-setup
  :config
  (js2-imenu-extras-setup))
```

gives me the byte compilation warning:

> the function ‘js2-imenu-extras-setup’ might not be defined at runtime.

I've spent hours but still wasn't able to silence the warning. I'm wondering whether it would be possible to solve it.