DataStreamPeer: add missing include <QDataStream>

When DataStreamPeer sources are used "standalone" ie. included to an external project:

    datastreampeer.cpp: In member function 'virtual void DataStreamPeer::processMessage(const QByteArray&)':
    datastreampeer.cpp:58:27: error: variable 'QDataStream stream' has initializer but incomplete type
    datastreampeer.cpp:59:23: error: incomplete type 'QDataStream' used in nested name specifier
    datastreampeer.cpp:62:28: error: incomplete type 'QDataStream' used in nested name specifier
    datastreampeer.cpp: In member function 'void DataStreamPeer::writeMessage(const QVariantList&)':
    datastreampeer.cpp:91:26: error: variable 'QDataStream msgStream' has initializer but incomplete type
    datastreampeer.cpp:92:26: error: incomplete type 'QDataStream' used in nested name specifier