[BUG] Fix CDash bug

 error: EXTENSION_FILE CMake variable points to a inexistent file or
  directory:
  /.../SlicerDiffusionComplexityMap-build/./DiffusionComplexityMap.json
Call Stack (most recent call first):
  /work/Preview/Slicer-0/Extensions/CMake/SlicerExtensionPackageAndUploadTarget.cmake:210 (slicerFunctionExtractExtensionDescriptionFromJson)


https://slicer.cdash.org/viewBuildError.php?buildid=3923529