SAML2: Fix declaration of 'xs' namespace.

$document->importNode(...) "helpfully" optimizes away the namespace
declaration for the 'xs' namespace. Fix this by avoiding to use that
function.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1636 44740490-163a-0410-bde0-09ae8108e29a