Use the correct icon in the restart dialog

It currently uses system-shutdown-symbolic. view-refresh-symbolic
is a better fit in this case.

https://bugzilla.gnome.org/show_bug.cgi?id=670687

js/ui/endSessionDialog.js

@@ -105,7 +105,7 @@ const restartDialogContent = {
     endDescription: _("Restarting the system."),
     confirmButtons: [{ signal: 'ConfirmedReboot',
                        label:  C_("button", "Restart") }],
-    iconName: 'system-shutdown-symbolic',
+    iconName: 'view-refresh-symbolic',
     iconStyleClass: 'end-session-dialog-shutdown-icon'
 };