qt5_add_big_resources is only available if using CMake 3.9 and later.
This amends cdccd0222b.
Task-number: QTBUG-55680
Task-number: QTBUG-75806
Change-Id: Ibba7af6ee7edfb226368937d543b7ec5cc93eb16
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Kai Koehne <kai.koehne@qt.io>