ScaledPlan: region subfield missing (needed for AD)

The `ScaledPlan` like e.g. the result of `plan_fft` has no field `region`, leading to an error when attempting to AD it via Zygote. Zygote (which depends on AbstractFFT) is requiring a `region` subfield.

Example
```Julia
begin
using Flux
using Zygote
using LinearAlgebra
using FFTW
end

N = 15
data = rand(Float64, N)

FT = plan_fft(data) # e.g. plan_dct / plan_idct would work
iFT= plan_ifft(data)

m = Chain(x->FT*x, Dense(N,N), x->iFT*x)
loss(x,y) = sum(abs2,m(x) .- y)
loss(data,data) # precompile

grads = gradient(() -> loss(data, data), params(m))
```