COMP: Do not build DWIConvert module if DICOM support is disabled

"DWIConvert.cxx" expects "itkDCMTKFileReader.h" header.
See https://github.com/BRAINSia/BRAINSTools/blob/ddf903b56fd6633945e1927b54393f7c65268dea/DWIConvert/DWIConvert.cxx#L47

Fixes #3370

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22412 3bd1e089-480b-0410-8dfb-8563597acbee