Use flags for drawing arrow in wxGTK DrawComboBoxDropButton()

This would seem to be required for drawing it correctly in the disabled
state, for example, but is still insufficient to fix its appearance, at
least with GTK 3.

See #22431.

src/gtk/renderer.cpp

@@ -550,7 +550,7 @@ wxRendererGTK::DrawComboBoxDropButton(wxWindow *win,
                                       int flags)
 {
     DrawPushButton(win,dc,rect,flags);
-    DrawDropArrow(win,dc,rect);
+    DrawDropArrow(win,dc,rect,flags);
 }
 
 // Helper used by GetCheckBoxSize() and DrawCheckBox().