cmTarget: Make GetSourceFiles populate an out-vector parameter.

In a future patch, this will also be populated with extra
sources from the linked dependencies.

Source/cmLocalVisualStudio7Generator.cxx

@@ -1381,7 +1381,8 @@ void cmLocalVisualStudio7Generator::WriteVCProjFile(std::ostream& fout,
 
   // get the classes from the source lists then add them to the groups
   this->ModuleDefinitionFile = "";
-  std::vector<cmSourceFile*>const & classes = target.GetSourceFiles();
+  std::vector<cmSourceFile*> classes;
+  target.GetSourceFiles(classes);
   for(std::vector<cmSourceFile*>::const_iterator i = classes.begin();
       i != classes.end(); i++)
     {