Add libxml2 exception for DESeq2

[JSchoenbachler]

@gaborcsardi sorry for another pull request, but I need this exception added as well.

Don't be scared by the amount of commits, if you look at them you'll see I forgot to fetch upstream before I committed the change so the commits are the following:

1. Add initial openssl exception. (Already merged)
2. Add libxml2 exception.
3. Revert commit in point 2.
4. Fetch upstream from this branch.
5. Re-add libxml2 exception.

The only thing changed in code is adding the libxml2 exception to DESeq2.

[gaborcsardi]

Thanks! Deployed: https://sysreqs.r-hub.io/pkg/DESeq2