Silence compiler by adding cosmetic virtual destructors.

[SVN r41544]

test/shared_ptr.cpp

@@ -129,6 +129,7 @@ shared_ptr<Y> factory(int n)
 // regressions from Nicodemus
     struct A
     {
+        virtual ~A() {}; // silence compiler warnings
         virtual int f() = 0;
         static int call_f(shared_ptr<A>& a) { return a->f(); }
     };