BUG: fix for 6720, source groups on vs6 not working

Source/cmLocalVisualStudio6Generator.cxx

@@ -358,7 +358,7 @@ void cmLocalVisualStudio6Generator
     sg->GetSourceFiles();
   // If the group is empty, don't write it at all.
         
-  if(sourceFiles.empty())
+  if(sourceFiles.empty() && sg->GetGroupChildren().empty())
     { 
     return; 
     }