fix (harmless for now) g++ warning about non-virtual dtor

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58733 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/persist.h

@@ -77,7 +77,7 @@ public:
     // deletes the associated wxPersistentObject
     void Unregister(void *obj);
 
-    
+
     // save/restore the state of an object
     //
     // these methods do nothing if DisableSaving/Restoring() was called
@@ -136,6 +136,10 @@ private:
         m_doRestore = true;
     }
 
+    // trivial but virtual dtor
+    virtual ~wxPersistenceManager();
+
+
     // helpers of Save/Restore()
     //
     // TODO: make this customizable by allowing
@@ -230,7 +234,7 @@ inline bool wxPersistentRegisterAndRestore(T *obj)
     wxPersistentObject * const pers = wxCreatePersistentObject(obj);
 
     return wxPersistenceManager::Get().RegisterAndRestore(obj, pers);
-                                            
+
 }
 
 #endif // _WX_PERSIST_H_