GtkStackSwitcher: center the button child

Set button child halign to GTK_ALIGN_CENTER since I need it to style
the needs-attention class, see:
https://bugzilla.gnome.org/show_bug.cgi?id=707153

gtk/gtkstackswitcher.c

@@ -137,6 +137,7 @@ rebuild_child (GtkWidget   *self,
 
   if (button_child)
     {
+      gtk_widget_set_halign (GTK_WIDGET (button_child), GTK_ALIGN_CENTER);
       gtk_widget_show_all (button_child);
       gtk_container_add (GTK_CONTAINER (self), button_child);
     }