Fix DBus compilation

Don't use C++11 nullptr in 5.6.

Change-Id: I57e9595b2e1cede995eed09878bf02ee30482659
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Alberto Mardegan <mardy@users.sourceforge.net>

src/dbus/qdbusserver.cpp

@@ -105,7 +105,7 @@ QDBusServer::~QDBusServer()
         }
         d->serverConnectionNames.clear();
     }
-    d->serverObject = nullptr;
+    d->serverObject = Q_NULLPTR;
     d->ref.store(0);
     d->deleteLater();
 }