Update the plugins.qmltypes meta data.

I forgot to do this when I introduced the QQmlWebSocketServer.

Change-Id: I7fd7354cf42f633973e0da71d854594627e2fb97
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>