[Development] Enabling private features for separate modules

[Alexander Volkov]

Hi,

I build qtwebengine as a separate module.
What is the right way to enable system-ffmpeg feature?
It's not auto-detected:
          "system-ffmpeg": {
              "label": "ffmpeg",
              "autoDetect": false,
              "condition": "libs.ffmpeg && features.system-opus && 
features.system-webp",
              "output": [ "privateFeature" ]
          }

Cheers,
Alexander.