`logunit` doesn’t support `Quantity{<:LogScaled}`

To me, it seems like this should work:
```julia
julia> u = u"dB/s"
dB s^-1

julia> unit(u)
s^-1

julia> unit(1u)
s^-1

julia> logunit(u)
dB

julia> logunit(1u)
ERROR: MethodError: no method matching logunit(::Quantity{Gain{Unitful.LogInfo{:Decibel, 10, 10}, :?, Int64}, 𝐓^-1, Unitful.FreeUnits{(s^-1,), 𝐓^-1, nothing}})
The function `logunit` exists, but no method is defined for this combination of argument types.
[…]
```