menu:style="radio" is used since 2010, update dtd to reality

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>

framework/dtd/menubar.dtd

@@ -17,6 +17,8 @@
  *   the License at http://www.apache.org/licenses/LICENSE-2.0 .
 -->
 
+<!ENTITY % menustyle "radio">
+
 <!ELEMENT menu:menubar (menu:menu+)>
 <!ELEMENT menu:menu (menu:menupopup)>
 <!ELEMENT menu:menupopup (menu:menuitem | menu:menuseparator | menu:menu)+>
@@ -34,4 +36,5 @@
 	menu:id CDATA #REQUIRED
 	menu:helpid CDATA #IMPLIED
 	menu:label CDATA #IMPLIED
+	menu:style %menustyle; #IMPLIED
 >