qfeatures: Allow QFileSystemModel without QFileSystemWatcher

As of 373e44d, QFileSystemModel can be built without QFileSystemWatcher
support, making this define incorrect.

Change-Id: I1c18dc4f323f94b43f4e49646911fd06b0f01f73
Reviewed-by: Sérgio Martins <sergio.martins@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>