kernel-doc: fix xml output mode

After Randy's patch fixing the HTML output in DOC: sections
(6b5b55f6) the same bug remained in XML
mode, this fixes it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>