This fixes build error with XCode 10.
XCode 10 build system (a.k.a "New Build System") requires all the files
that are generated by scripts and used later on build to be explicitly
defined as output files.
Task-number: QTBUG-71035
Pick-to: 6.0 6.1 5.15 5.12
Change-Id: Ibec39eee53b0cb3acecf592f1ca53c04b9975cad
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>