QNetworkAccessManager: optimize QNetworkAccessAuthenticationManager allocation

Use QSharedPointer<T>::create(), which co-locates the refcount with the payload
in a single memory allocation, instead of QSharedPointer<T>(new T), which causes
two allocations.

Change-Id: I58196b6390dcc9bd52417e279fd9e000ab9ee9cc
Reviewed-by: Richard J. Moore <rich@kde.org>

src/network/access/qnetworkaccessmanager_p.h

@@ -92,7 +92,7 @@ public:
           initializeSession(true),
 #endif
           cookieJarCreated(false),
-          authenticationManager(new QNetworkAccessAuthenticationManager)
+          authenticationManager(QSharedPointer<QNetworkAccessAuthenticationManager>::create())
     { }
     ~QNetworkAccessManagerPrivate();