make destructors virtual to fix warnings

gfx/src/gtk/nsScreenGtk.h

@@ -32,7 +32,7 @@ class nsScreenGtk : public nsIScreen
 public:
   //nsScreenGtk ( GDHandle inScreen );
   nsScreenGtk ( );
-  ~nsScreenGtk();
+  virtual ~nsScreenGtk();
 
   NS_DECL_ISUPPORTS
   NS_DECL_NSISCREEN

gfx/src/gtk/nsScreenManagerGtk.h

@@ -33,7 +33,7 @@ class nsScreenManagerGtk : public nsIScreenManager
 {
 public:
   nsScreenManagerGtk ( );
-  ~nsScreenManagerGtk();
+  virtual ~nsScreenManagerGtk();
 
   NS_DECL_ISUPPORTS
   NS_DECL_NSISCREENMANAGER