IO/XML: Add missing <string> include

Now that <sstream> is not included we need <string> to get std::string
when compiling Java wrapper sources.

Change-Id: I5b36410f8bdb2e13c497b0adaa11e95142fd56a0

IO/XML/vtkXMLReader.h

@@ -24,6 +24,8 @@
 #include "vtkIOXMLModule.h" // For export macro
 #include "vtkAlgorithm.h"
 
+#include <string>
+
 class vtkAbstractArray;
 class vtkCallbackCommand;
 class vtkDataArraySelection;