libxml2 rev bumps

[Joshua Root]

The major library version of libxml2 has changed in the update that was 
just committed, so everything linked with it needs to be rebuilt. All 
declared dependents have been rev bumped already, but please keep an eye 
out for any ports caught by rev-upgrade which may be using it without 
declaring a dependency. These will need to add the dependency and bump 
the revision.

- Josh