librem5: update-dialog: Don't show a double back button

Make the headerbar not show its "close" back button when the additional
back button is visible.

src/gs-update-dialog.ui

@@ -10,6 +10,7 @@
     <property name="use_header_bar">1</property>
     <child internal-child="headerbar">
       <object class="GtkHeaderBar">
+        <property name="show_close_button" bind-source="button_back" bind-property="visible" bind-flags="sync-create|invert-boolean"/>
         <child>
           <object class="GtkButton" id="button_back">
             <property name="visible">True</property>