security/zeek: Fix build with clang 19

    https://github.com/zeek/zeek/issues/3994
    https://github.com/zeek/zeek/pull/3997

    Clang 19 with libc++ started failing to compile because the
    default implementation of std::char_traits was removed, making
    uses of std::char_traits<unsigned char> invalid (by consequence,
    also std::basic_string<unsigned char>).