cmake: fix docbook dependency

cmake: fix docbook dependency

After changing an asciidoc source file, the html files were not rebuilt
via 'ninja developer_guides'. It turns out that the
developer_guide_docbook target has an order-only dependency on
developer-guide.xml (and not the developer-guide.xml dependencies).

Fix this by adding an explicit dependency on the generated .xml file.

Change-Id: I4bdc0ebca8909caaab9cc0797cc35cc6260bfe43
Reviewed-on: https://code.wireshark.org/review/14065
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Graham Bloice <graham.bloice@trihedral.com>