menubar definitions using  menu:style="radio" attribute were
introduced with df4017a8 / the other
commits of that cws, but the dtd wasn't adjusted accordingly

Change-Id: I7c7e1b8116800ee20cfede431ed90ef8f213cce0
Reviewed-on: https://gerrit.libreoffice.org/60276
Tested-by: Jenkins
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>