Libxml++ update for Cyclus 1.6

[RhysMacMillan]

libxml++ was removed as a dependency when Cyclus was updated to version 1.6. This issue is prevalent in older archetypes and I'll be opening issues and pull requests to address this in the ones I find.
This issue can be closed by a pull request that makes this change to CMakeLists.txt.

[maxschalz]

Thanks for mentioning this! I will make sure to change this in all my Cyclus repo

[RhysMacMillan]

I'm working on a PR now, if you'd like to list others I should change I could do those as well.