Add missing inclusion of sstream/string headers

Needed for std::ostringstream and std::string.

On other configurations it may work due to transitive includes, but on mine the lack of the sstream header gave:

[ 77%] Building CXX object IO/Geometry/CMakeFiles/IOGeometry.dir/vtkGLTFWriterUtils.cxx.o
cd /buildbot/b-vtk9/build/debian/build/IO/Geometry && /usr/bin/c++  -DIOGeometry_EXPORTS -DVTK_IN_VTK -Dkiss_fft_scalar=double -I/buildbot/b-vtk9/build/debian/build/IO/Geometry -I/buildbot/b-vtk9/build/IO/Geometry -I/buildbot/b-vtk9/build/debian/build/Common/Core -I/buildbot/b-vtk9/build/Common/Core -I/buildbot/b-vtk9/build/debian/build/Common/DataModel -I/buildbot/b-vtk9/build/Common/DataModel -I/buildbot/b-vtk9/build/debian/build/Common/Math -I/buildbot/b-vtk9/build/Common/Math -I/buildbot/b-vtk9/build/debian/build/ThirdParty/kissfft/vtkkissfft -I/buildbot/b-vtk9/build/ThirdParty/kissfft/vtkkissfft -I/buildbot/b-vtk9/build/debian/build/Common/Transforms -I/buildbot/b-vtk9/build/Common/Transforms -I/buildbot/b-vtk9/build/debian/build/Common/ExecutionModel -I/buildbot/b-vtk9/build/Common/ExecutionModel -I/buildbot/b-vtk9/build/debian/build/IO/Core -I/buildbot/b-vtk9/build/IO/Core -I/buildbot/b-vtk9/build/debian/build/IO/Legacy -I/buildbot/b-vtk9/build/IO/Legacy -I/buildbot/b-vtk9/build/debian/build/Common/Misc -I/buildbot/b-vtk9/build/Common/Misc -I/buildbot/b-vtk9/build/debian/build/Common/System -I/buildbot/b-vtk9/build/Common/System -I/buildbot/b-vtk9/build/debian/build/Filters/General -I/buildbot/b-vtk9/build/Filters/General -I/buildbot/b-vtk9/build/debian/build/Filters/Core -I/buildbot/b-vtk9/build/Filters/Core -I/buildbot/b-vtk9/build/debian/build/Filters/Hybrid -I/buildbot/b-vtk9/build/Filters/Hybrid -I/buildbot/b-vtk9/build/debian/build/Filters/Geometry -I/buildbot/b-vtk9/build/Filters/Geometry -I/buildbot/b-vtk9/build/debian/build/Imaging/Core -I/buildbot/b-vtk9/build/Imaging/Core -I/buildbot/b-vtk9/build/debian/build/IO/Image -I/buildbot/b-vtk9/build/IO/Image -I/buildbot/b-vtk9/build/debian/build/Rendering/Core -I/buildbot/b-vtk9/build/Rendering/Core -isystem /buildbot/b-vtk9/build/debian/build/Utilities/KWIML -isystem /buildbot/b-vtk9/build/Utilities/KWIML -isystem /buildbot/b-vtk9/build/debian/build/Utilities/KWSys -isystem /buildbot/b-vtk9/build/Utilities/KWSys -isystem /buildbot/b-vtk9/build/debian/build/ThirdParty/kissfft -isystem /buildbot/b-vtk9/build/ThirdParty/kissfft -isystem /buildbot/b-vtk9/build/debian/build/ThirdParty/jsoncpp -isystem /buildbot/b-vtk9/build/ThirdParty/jsoncpp -isystem /usr/include/jsoncpp -isystem /buildbot/b-vtk9/build/debian/build/ThirdParty/zlib -isystem /buildbot/b-vtk9/build/ThirdParty/zlib  -g -O2 -fdebug-prefix-map=/buildbot/b-vtk9/build=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2  -fPIC -fvisibility=hidden -fvisibility-inlines-hidden   -std=c++11 -o CMakeFiles/IOGeometry.dir/vtkGLTFWriterUtils.cxx.o -c /buildbot/b-vtk9/build/IO/Geometry/vtkGLTFWriterUtils.cxx
/buildbot/b-vtk9/build/IO/Geometry/vtkGLTFWriterUtils.cxx: In static member function ‘static void vtkGLTFWriterUtils::WriteBufferAndView(vtkDataArray*, const char*, bool, Json::Value&, Json::Value&)’:
/buildbot/b-vtk9/build/IO/Geometry/vtkGLTFWriterUtils.cxx:57:24: error: aggregate ‘std::ostringstream toString’ has incomplete type and cannot be defined
   57 |     std::ostringstream toString;
      |                        ^~~~~~~~

@danlipsa