Fix -Wweak-vtables warning

Change-Id: I0cb5916093edf08d72add93573b828021e86256a
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/network/access/qnetworkcookiejar/tst_qnetworkcookiejar.cpp

@@ -60,12 +60,15 @@ private slots:
 class MyCookieJar: public QNetworkCookieJar
 {
 public:
+    ~MyCookieJar() override;
     inline QList<QNetworkCookie> allCookies() const
         { return QNetworkCookieJar::allCookies(); }
     inline void setAllCookies(const QList<QNetworkCookie> &cookieList)
         { QNetworkCookieJar::setAllCookies(cookieList); }
 };
 
+MyCookieJar::~MyCookieJar() = default;
+
 void tst_QNetworkCookieJar::getterSetter()
 {
     MyCookieJar jar;