I couldn’t find a 5 argument method of `mul!`

in FFTW.jl for `FFTW.rFFTWPlan`

and its siblings. Would it be reasonable to add such methods? In particular, I would like to be able to do something like

```
julia> using LinearAlgebra, FFTW
julia> x = Vector(range(0., 1., length=16));
julia> rfft_plan = plan_rfft(x);
julia> y = rfft_plan * x; y .= 1;
julia> mul!(y, rfft_plan, x);
julia> mul!(y, rfft_plan, x, 1.0, 0.5) # this yields a MethodError
```