Variable-length arrays are not compatible with MSVC

[thomwiggers]

In the category "Microsoft compilers suck", such as previously seen in #14, the use of variable-length arrays is not supported by Microsoft compilers. This means even the ref implementation doesn't work on Windows.

[thomwiggers]

As used in e.g. https://github.com/thomwiggers/sphincsplus/blob/master/ref/thash_haraka_robust.c#L16-L17