multimedia/gstreamer1-qt: fix build on current by using c++11

/wrkdirs/usr/ports/multimedia/gstreamer1-qt/work/qt-gstreamer-1.2.0/src/QGlib/connect.cpp:395:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
register bool ok = (m_handlerIdInRemoval != handlerId);
^~~~~~~~~