mark all dtors which are virtual because base class dtor is virtual explicitly virtual themselves

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@41020 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/fs_mem.cpp

@@ -49,7 +49,7 @@ class MemFSHashObj : public wxObject
             InitTime();
         }
 
-        ~MemFSHashObj()
+        virtual ~MemFSHashObj()
         {
             delete[] m_Data;
         }