codecvt_null.hpp unconditional symbols leakage

[claimred]

Why does codecvt_null.hpp file declares symbols with BOOST_SYMBOL_EXPORT instead of BOOST_ARCHIVE_DECL?

https://github.com/boostorg/serialization/blob/develop/include/boost/archive/codecvt_null.hpp#L69

https://github.com/boostorg/serialization/blob/develop/include/boost/archive/detail/decl.hpp#L26

It leads to symbols leakage in DLLs statically linking boost.archive.

[robertramey]

It's probably a mistake as I had lots of difficulty with symbol export macros. Have you made this change and run the tests to verify that making this change doesn't break anything else.