etl::forward_link does not have constexpr ctor anymore

[diggit]

Hi,
since commit c3c12da , which added user defined ctors to etl::forward_link not marked with constexpr, it's not possible to construct it in compile time. Thus any child class also can't have constexpr ctors.

I know it's not easy to handle constexpr stuff in multi standard library, but here it should be only about adding appropriate ETL_CONSTEXPR macro prefix or am I wrong?

[jwellbelove]

I'll check whether constexpr constructors are possible.