The VTK standard headers include <iosfwd> and make std::istringstream
available for use as an incomplete type.  That is all vtkXMLReader.h
needs.  Remove the extra <sstream> include.

In particular, by including it before the VTK headers it causes system
headers to be included with inconsistent Large File Support definitions
on AIX, leading to runtime shared library unresolved symbols.  Remove
the inclusion to fix this.

Change-Id: I78ec9efae24c8f721f079ab8946afcde413fe17f