vtkFFT.txx: add needed but missing typename keyword

This fixes a compilation error on Intel Classic compilers.

Common/Math/vtkFFT.txx

@@ -327,7 +327,7 @@ vtkSmartPointer<vtkFFT::vtkScalarNumberArray> vtkFFT::Csd(vtkScalarNumberArray*
   vtkSMPTools::Fill(averageRange.begin(), averageRange.end(), 0.0);
 
   auto resRange = vtk::DataArrayTupleRange(result);
-  using ConstTupleRef = decltype(resRange)::ConstTupleReferenceType;
+  using ConstTupleRef = typename decltype(resRange)::ConstTupleReferenceType;
   for (unsigned int i = 0; i < shape[0]; ++i)
   {
     auto begin = resRange.cbegin() + i * shape[1];