fix for doxygen 1.8.7

Earlier versions apparently used HTML as default if no output format was
specified. 1.8.7 no longer does that.

Change-Id: Ia8f908f54b344530e1b50e8800c032f782fb36e9
(cherry picked from commit 2e9a77aa2d8e83f6296cd9204759d18b0ec5f0b5)
Reviewed-on: https://gerrit.libreoffice.org/9362
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>