Missing include in otbStackIndiceFunctor.h

Summary

IndicesStackFunctor uses itk::VariableLengthVector but the file itkVariableLengthVector.h is not included. Depending on how otbStackIndiceFunctor.h is included this might not cause a compilation error, but in my case when I try to build the tests I get the following error:

[ 50%] Building CXX object Modules/Radiometry/Indices/CMakeFiles/OTBIndicesHeaderTest1.dir/test/OTBIndicesHeaderTest1.cxx.o
In file included from /Datas/Code/OTB/SuperBuild/OTB/build/Modules/Radiometry/Indices/test/OTBIndicesHeaderTest1.cxx:23:0:
/Datas/Code/OTB/otb/Modules/Radiometry/Indices/include/otbIndicesStackFunctor.h:54:22: error: ‘itk’ does not name a type
   using OutputType = itk::VariableLengthVector<typename IndiceType::OutputType>;
                      ^
/Datas/Code/OTB/otb/Modules/Radiometry/Indices/include/otbIndicesStackFunctor.h:75:19: error: ‘OutputType’ has not been declared
   void operator()(OutputType& out, const PixelType& in) const
                   ^
/Datas/Code/OTB/otb/Modules/Radiometry/Indices/include/otbIndicesStackFunctor.h: In member function ‘void otb::Functor::IndicesStackFunctor<TIndice>::operator()(int&, const PixelType&) const’:
/Datas/Code/OTB/otb/Modules/Radiometry/Indices/include/otbIndicesStackFunctor.h:80:14: error: invalid types ‘int[size_t {aka long unsigned int}]’ for array subscript
       out[idx] = (*indice)(in);
              ^

This MR adds the missing header

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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