Cannot link against t8code with VTK

Currently linking against t8code with VTK does not work.
I get the following output from CMake:

```
cmake
  The link interface of target "T8CODE::T8" contains:

    VTK::IOXML

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.
```