Fix gtk_widget_get_allocation()

- add it to the header
- add the symbol to gtk.symbols
- fix coding style and check for allocation != NULL

gtk/gtkwidget.h

@@ -599,6 +599,9 @@ void                  gtk_widget_set_child_visible      (GtkWidget    *widget,
 gboolean              gtk_widget_get_child_visible      (GtkWidget    *widget);
 GdkWindow*            gtk_widget_get_window             (GtkWidget    *widget);
 
+void                  gtk_widget_get_allocation         (GtkWidget     *widget,
+                                                         GtkAllocation *allocation);
+
 gboolean   gtk_widget_child_focus         (GtkWidget           *widget,
                                            GtkDirectionType     direction);
 gboolean   gtk_widget_keynav_failed       (GtkWidget           *widget,