bug#32975: Emacs 26.1; GTK Warnings

[martin rudalics]

> So I donʼt think we should be calling GTK functions on scrollbars that
> are not GTK widgets (Iʼm stunned we've never seen this before).

Setting the background color on a GTK build without GTK scroll bars
seems pretty rare to me.  In addition, one usually has to look at the
console to see the warnings.

> This
> fixes it for me. Martin?

Do we want to skip xg_set_widget_bg too?

martin