This fixes build errors with Xcode 10.
Xcode 10 build system (a.k.a "New Build System") needs to know the input files
in order to build a correct dependency graph. Especially when a build is not run
for the first time and files changed in-between.
Task-number: QTBUG-71035
Pick-to: 6.0 6.1 5.15 5.12
Change-Id: If8fbad3a1915add9b35c79131b03cdbe6b7ac06d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>