COMP: Fix non-virtual destructor warning

This gives cmFileCopier a virtual destructor since it has virtual
methods.  While we never actually delete through a base pointer (or
dynamically at all), the compiler doesn't know and warns anyway.

Source/cmFileCommand.cxx

@@ -922,6 +922,7 @@ struct cmFileCopier
     Doing(DoingNone)
     {
     }
+  virtual ~cmFileCopier() {}
 
   bool Run(std::vector<std::string> const& args);
 protected: