[XForms] Fix accessors interface for group. Bug 331197, r=smaug+doronr

extensions/xforms/resources/content/xforms.css

@@ -106,6 +106,11 @@ alert {
   display: none;
 }
 
+/* group */
+group {
+  -moz-binding: url('chrome://xforms/content/xforms.xml#xformswidget-base');
+}
+
 /* output widgets */
 html|*:root output {
   -moz-binding: url('chrome://xforms/content/xforms-xhtml.xml#xformswidget-output');