otherwise when compiling with the new seastar, which removed
`#include <variant>` from `std-compat.hh`, the {mode}-headers
target would fail to build, like:
```
./data_dictionary/storage_options.hh:34:29: error: no template named 'variant' in namespace 'std'
10:45:15 using value_type = std::variant<local, s3>;
10:45:15 ~~~~~^
10:45:15 ./data_dictionary/storage_options.hh:35:5: error: unknown type name 'value_type'; did you mean 'std::_Bit_const_iterator::value_type'?
10:45:15 value_type value = local{};
10:45:15 ^~~~~~~~~~
10:45:15 std::_Bit_const_iterator::value_type
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#18921
f58f6dfe20