added _change_notify in Control::set_scale to fix issue #48936

[phil-shenk]

Control::set_scale appeared to be missing _change_notify("rect_scale");, which prevented the inspector from receiving updates as controls were scaled.
this adds the _change_notify line to match Control::set_pivot_offset, Control::set_rotation, etc.

should not apply to 4.0+ :

Since change_notify() was removed in favor of polling in the master branch, this issue is likely only present in the 3.x branch.
thank you Calinou

Bugsquad edit: Fix #48936

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉