Always use gint for border_width

This avoids an evil trap when doing MAX (..., ... - 2 * border_width)
and the expression on the right gets promoted to unsigned, instead
of going negative as you would expect.
https://bugzilla.gnome.org/show_bug.cgi?id=699633

gtk/gtknotebook.c

@@ -2459,7 +2459,7 @@ gtk_notebook_size_allocate (GtkWidget     *widget,
 
   if (priv->children)
     {
-      guint border_width = gtk_container_get_border_width (GTK_CONTAINER (widget));
+      gint border_width = gtk_container_get_border_width (GTK_CONTAINER (widget));
       GtkNotebookPage *page;
       GtkAllocation child_allocation;
       GList *children;