add Loadpath to printers.mllib

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16492 85f007b7-540e-0410-9357-904b9bb8a0f7
Xml_datatype.mli ships the xml type

This breaks the dependency of Xml_parser, Xml_printer and Xml_utils
on Serialize.

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/coq/trunk@16493 85f007b7-540e-0410-9357-904b9bb8a0f7