Fix warning for -no-feature-graphicsview

Change-Id: I3229fccd2f837b8b8b4da2a149d4584cb4a54dab
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Tasuku Suzuki <tasuku.suzuki@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/widgets/util/qflickgesture.cpp

@@ -292,6 +292,8 @@ protected:
                 qFGDebug() << "QFG: ungrabbing" << grabber;
                 grabber->ungrabMouse();
             }
+#else
+            Q_UNUSED(flags);
 #endif // QT_NO_GRAPHICSVIEW
 
             if (me) {