c++: Add static in g++.dg/warn/Waddress-5.C

While reviewing some other changes I noticed that this test talks
about 'sf' being static, but it wasn't actually marked as such.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Waddress-5.C: Make sf static.

gcc/testsuite/g++.dg/warn/Waddress-5.C

@@ -12,7 +12,7 @@ struct A
   virtual void vf ();
   virtual void pvf () = 0;
 
-  void sf ();
+  static void sf ();
 
   int *p;
   int a[2];