constrain vectorizable std::complex specializations

[mattkretz]

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2663r3.html#word_general adds all possible std::complex specializations to the set of vectorizable types. I don't think that's correct. At the very least the T in std::complex<T> must itself be a vectorizable type. But something more elaborate might be necessary because of how std::complex only reliably works for floating-point types.