COMP: Build SlicerBaseCLI only if VTK is found.

This CMakeLists can be configured by projects others than Slicer
(e.g. TubeTK) that may not have VTK.
This time (see r22122 and r22125), the condition on SlicerBaseCLI_SRC
is correct.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22126 3bd1e089-480b-0410-8dfb-8563597acbee