Fix build with current KF6 KIO
Commit 57c6cc875f47057255ccb52891a24e0e518aa62c in KIO changes the API of KIO::FileSystemFreeSpaceJob.
Adapt dolphin code using it to the new API.
Commit 57c6cc875f47057255ccb52891a24e0e518aa62c in KIO changes the API of KIO::FileSystemFreeSpaceJob.
Adapt dolphin code using it to the new API.