COMP: Fix CTK compilation error expecting moc_ file to have cpp/cxx extension

This commit fixes issue introduced by r23100

$ git shortlog 909073..2f1ff44 --no-merges
Jean-Christophe Fillion-Robin (1):
      Fix compilation error introduced by 130deabb4. See #449

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23101 3bd1e089-480b-0410-8dfb-8563597acbee