Add missing @class attribute to <navref> in chunking phase

A `<topicset>` element inherits a `@chunk` attribute value of `to-navigation` from
the DTD. In the chunking phase, elements with `@chunk="to-navigation"` are
converted into a `<navref>` element. Currently, this `<navref>` element gets no
`@class` attribute value.

With the Eclipse Help transtype, this `<navref>` element should be converted into
a `<link>` element in the Eclipse Help TOC file, but since `map2eclipseImpl.xsl`
(correctly) matches against the `@class` attribute instead of the element name, it
ignores the `<navref>` element and the `<link>` element is never created. Adding the
missing `@class` attribute to the `<navref>` element solves the problem.