progress-info-widget: Make button round again

The button currently looks like a vertical pill, not a circle.

This is a regression from a2d5dafa05

Not using the GtkButton image API apparently requires us to add the image-button style class ourselves.

src/resources/ui/nautilus-progress-info-widget.ui

@@ -52,6 +52,7 @@
         <property name="valign">center</property>
         <property name="margin_start">20</property>
         <style>
+          <class name="image-button"/>
           <class name="circular"/>
         </style>
         <child>