QGst/caps: compilation fix from https://bugs.kde.org/show_bug.cgi?id=406676#c2
QGst/caps: compilation fix from https://bugs.kde.org/show_bug.cgi?id=406676#c2

Because the macro version of gst_caps_copy() confuses the C++ compiler

GStreamer/qt-gstreamer,GStreamer/qt-gstreamer,GStreamer/qt-gstreamer