Add [[maybe_unused]] attribute to an unused variable

Otherwise, it could break the unity build.

Pick-to: 6.5
Task-number: QTBUG-109394
Change-Id: Ic1b3ac51b8a9e8bb93a3b1a4d6717fa45099e91b
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/widgets/kernel/qgesturemanager_p.h

@@ -64,13 +64,16 @@ private:
     QSet<QGesture *> m_activeGestures;
     QSet<QGesture *> m_maybeGestures;
 
-    enum State {
+    QT_WARNING_PUSH
+    QT_WARNING_DISABLE_GCC("-Wattributes")
+    [[maybe_unused]] enum State {
         Gesture,
         NotGesture,
         MaybeGesture // this means timers are up and waiting for some
                      // more events, and input events are handled by
                      // gesture recognizer explicitly
     } state;
+    QT_WARNING_POP
 
     struct ObjectGesture
     {