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>).