meson: allow building plugins with GTK 4 examples
requested to merge m.tretter/gst-plugins-rs:allow-building-plugins-with-gtk4-examples-without-gtk4 into main
Only the examples of the fallbackswitch, livesync, and togglerecord plugins require the gtk, gio, and gst-plugin-gtk4 features. The plugins themselves don't actually have a dependency on GTK.
Only add the features (and examples) if the examples are actually enabled to allow building these plugins without the GTK dependency.