Fix test after proper rule enforced by gnat on taft types.

From-SVN: r146231

gcc/testsuite/gnat.dg/ref_type.adb

@@ -2,7 +2,6 @@
 --  { dg-do compile }
 
 package body ref_type is
-  type T is tagged null record;
   procedure Print (X : T) is                                   
   begin                                                        
      null;

gcc/testsuite/gnat.dg/ref_type.ads

 package ref_type is
 private
-   type T is tagged;
+   type T is tagged null record;
    procedure Print (X : T);
 end ref_type;