meson: Add qt5 feature

I'm a bit wary of adding this while this meson bug is open.

mentioned in merge request gst-plugins-good!729 (merged)

I'm not involved in this thread, but Jussi made it clear it's not a bug (not sure why he leaves things open when he disagree it's a bug). For this specific case, I don't see much value to have per repo control of this settings when using gst-build. Each plugins can be controlled, so you can still have per plugin control, remains the uninstalled examples, which can also be controlled through the "example" option. Though the implementation might still crash calling into qt5, but I guess if it's a problem to someone it can be fixed later.

An enhancement over this would be to visit both repos and make sure to have a failure when setting that to enable and QT5 could not be found, right now the implementation is close to a boolean, but I think it can also be fixed later.