NIFTI makes IOImage module depend on vtkzlib.

This dependency has to be stated explicitly.  This error wasn't caught
immediately because, on most sytems, vtkzlib is linked transitively via
vtktiff and vtkpng.

Change-Id: I36408557a7eed266fe05cdc891fc75dc1259502e
Merge topic 'nifti-zlib-depends' into master

1f777440 NIFTI makes IOImage module depend on vtkzlib.