Hi all,
I found the solution, which is extremely easy, but just in case you have a similar problem:
you only need
<xs:import namespace = "http://www.w3.org/XML/1998/namespace"></xs:import>Do not provide a schemaLocation, this schema is predefined in Tamino. I do not have a reference, but I just applied it and it worked 
In case you parse your schema outside of Tamino with another tool, this tool might complain about not finding the xml.xsd, just ignore it
Thorsten