When we skip a plugin due to missing dependencies, we should
output which dependencies are missing so that people can easily
find out what to add to their .pro files.

Task-number: QTBUG-34586
Change-Id: I64e1687e4ad67165b0d8708e1e1fcedc10883515
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>