qabstractitemmodel.cpp: Fix warnings.

Change-Id: I10ef89e2fab31c7f1c9ac67b2234f509fb3f28ab
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

src/corelib/itemmodels/qabstractitemmodel.cpp

@@ -1766,6 +1766,11 @@ bool QAbstractItemModel::canDropMimeData(const QMimeData *data, Qt::DropAction a
                                          int row, int column,
                                          const QModelIndex &parent) const
 {
+    Q_UNUSED(data)
+    Q_UNUSED(action)
+    Q_UNUSED(row)
+    Q_UNUSED(column)
+    Q_UNUSED(parent)
     return true;
 }