COMP: Ensure DCMTK libraries are packaged.

This commit fixes a regression introduced in r23413 where call to
"find_package(DCMTK ..)" have been moved in the sub-project where
it was explicitly needed. By calling, "find_package(DCMTK ..)", it
now ensures that the variable "DCMTK_LIBRARIES" will be set.

Note that ideally DCMTK libraries could be installed leveraging the
fact the project is cmake'ified by installing the expected components.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23430 3bd1e089-480b-0410-8dfb-8563597acbee