QFileSystemModel: replace a TODO with QT{6,7}Only

So that it's not forgotten come Qt7.

Amends 19258608e9ea02043ce9b53d4a9c99700ce49c1b.

Change-Id: I323f2d2738a295faa1e118c3ea7c8a5b6c6d0ac7
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit ef8e1e5861292514a676c5fed4d670fb6ecb887d)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/itemmodels/qfilesystemmodel.h

@@ -32,13 +32,19 @@ Q_SIGNALS:
     void directoryLoaded(const QString &path);
 
 public:
-    // ### Qt 7: renumber these values to be before Qt::UserRole comment.
     enum Roles {
         FileIconRole = Qt::DecorationRole,
+        QT7_ONLY(
+        FilePathRole = Qt::UserRole - 4,
+        FileNameRole = Qt::UserRole - 3,
+        FilePermissions = Qt::UserRole - 2,
+        )
         FileInfoRole = Qt::UserRole - 1,
+        QT6_ONLY(
         FilePathRole = Qt::UserRole + 1,
         FileNameRole = Qt::UserRole + 2,
         FilePermissions = Qt::UserRole + 3,
+        )
     };
 
     enum Option