fix #1471

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@17251 29f149b2-0c63-11e1-8f5c-a347f6a9acd0
fixing missing attribute in xsd

git-svn-id: http://svn.code.sf.net/p/sumo/code/trunk/sumo@17252 29f149b2-0c63-11e1-8f5c-a347f6a9acd0