IO/XML: Fix HeaderTest for this module

Comment reason for <string> inclusion.

Change-Id: I7e0b1d7bd52a3ee66f5d078f330520bb7e6461a2

IO/XML/vtkXMLReader.h

@@ -24,7 +24,7 @@
 #include "vtkIOXMLModule.h" // For export macro
 #include "vtkAlgorithm.h"
 
-#include <string>
+#include <string> // for std::string
 
 class vtkAbstractArray;
 class vtkCallbackCommand;