Fix xrefs to preface topics #1732

[Fixtures](https://github.com/eerohele/dita-ot-issues/tree/master/fixtures/1732).

The "commonattributes" template wasn't called for preface topics, which
meant that no @id attribute was added for them when generating the
output XSL-FO markup.

This should also probably be merged into the 1.8.5 branch if possible?