COMP: Fix visibility warning in DiffusionTensor3DRead

Remove warning: ‘itk::DiffusionTensor3DRead<float>’ declared with greater
visibility than the type of its field ‘itk::DiffusionTensor3DRead<float>::m_Reader’

Due to the fact that itkDiffusionTensor3DRead was declaring a private
variable of type itkImageFileReader<DiffusionImageType>::Pointer and that
this class was declared with ITK_ABI_EXPORT, we had to add ITK_ABI_EXPORT
to the declaration of itkDiffusionTensor3DRead to remove the previously
described warning when ResampleDTIVolume was compiled (shared object).

From: Francois Budin <fbudin@unc.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23139 3bd1e089-480b-0410-8dfb-8563597acbee