Fix build not working anymore since demo app merge
Disable the GTK+ demo application, so we don't need to install GTK+. It failed a couple of days ago, but as GitLab still sends us failure messages when things work, that's why it wasn't caught.
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Disable the GTK+ demo application, so we don't need to install GTK+. It failed a couple of days ago, but as GitLab still sends us failure messages when things work, that's why it wasn't caught.