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/gtkframe.c

@@ -816,7 +816,7 @@ gtk_frame_real_compute_child_allocation (GtkFrame      *frame,
   GtkAllocation allocation;
   GtkBorder padding;
   gint top_margin;
-  guint border_width;
+  gint border_width;
 
   gtk_widget_get_allocation (widget, &allocation);
   get_padding_and_border (frame, &padding);