Fix a missing include of <string>
This somehow only got catched by paraview windows buildbots. https://open.cdash.org/index.php?compare1=61&filtercount=1&field1=revision&project=ParaView&showfilters=0&limit=100&value1=239f6243399b3c5c3aef5239aceb5597dc6f083d&showfeed=0
Edited by Mathieu Westphal (Kitware)