add missing emit_changed() to set update mode function

(cherry picked from commit 51db86d7895db95c1be2562b0a48dc75b520c77d)

scene/resources/animation.cpp

@@ -1870,6 +1870,7 @@ void Animation::value_track_set_update_mode(int p_track, UpdateMode p_mode) {
 
 	ValueTrack *vt = static_cast<ValueTrack *>(t);
 	vt->update_mode = p_mode;
+	emit_changed();
 }
 
 Animation::UpdateMode Animation::value_track_get_update_mode(int p_track) const {