autoclose: fixed deprecated gtk call for gtk3

[lpaulsen93]

This fixes a deprecation warning in the autoclose plugin for gtk3. No functional changes.

A little side note:
I first thought I could use gtk_container_add() on gtk 2 and 3 because of this deprecation notice in the gtk documentation:

gtk_scrolled_window_add_with_viewport has been deprecated since version 3.8 and should not be used in newly-written code.
gtk_container_add() will automatically add a GtkViewport if the child doesn’t implement GtkScrollable.

But the function only works that way under gtk3. A test with gtk2 revealed that the function does not work that way with the result that in this case the scroll bar was missing in the autoclose configuration dialog.