Be explicit about the ibtool output format.

A matter of style, just to be certain.

Change-Id: Ie8e5be27b2997ed7d3e59c5924f153a82f68c399
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@theqtcompany.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>