gstreamer1.0-plugins-good: Add Qt6 QML plugin support

gstreamer1.0-plugins-good: Add Qt6 QML plugin support

GStreamer 1.26 supports both Qt5 and Qt6 QML video sink plugins.
Add PACKAGECONFIG[qt6] to enable the Qt6 plugin alongside the
existing Qt5 support.

The Qt5 plugin builds ext/qt/ directory, while Qt6 builds ext/qt6/.
Both can coexist and are controlled by separate meson options
(-Dqt5 and -Dqt6).

Signed-off-by: Telukula Jeevan Kumar Sahu <j-sahu@ti.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>