`@redef_print` doesn't work with parametric types

[FedericoStra]

julia> using RedefStructs

julia> @redef_print struct S{T}
           x::T
       end
ERROR: ArgumentError: method definition for show_datatype at /home/fstra/.julia/dev/RedefStructs/src/RedefStructs.jl:88 has free type variables
Stacktrace:
 [1] top-level scope
   @ ~/.julia/dev/RedefStructs/src/RedefStructs.jl:91