webrtc: janus: use 'member' when defining janus-state property

It actually works when passing the type as well.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/2244>

net/webrtc/src/webrtcsink/imp.rs

@@ -6001,8 +6001,8 @@ pub(super) mod janus {
          */
         #[property(
         name = "janus-state",
-        // FIXME: can't use `member =` with enums: https://github.com/gtk-rs/gtk-rs-core/issues/1338
-        get = |self_: &Self| self_.state.lock().unwrap().janus_state,
+        get,
+        member = janus_state,
         type = JanusVRSignallerState,
         blurb = "The current state of the signaller",
         builder(JanusVRSignallerState::Initialized)