gs-update-dialog.ui: Add left and right margin to os_update_box

The current os_update_box looks a bit weird, because the left and right
margin is missing.

I've added a 18px left&right margin. Now it looks better.

#317

src/gs-update-dialog.ui

@@ -260,6 +260,8 @@
                     <property name="visible">True</property>
                     <property name="orientation">vertical</property>
                     <property name="margin_bottom">18</property>
+                    <property name="margin_start">18</property>
+                    <property name="margin_end">18</property>
                   </object>
                 </child>
               </object>