Update changelog of boost-1.69 about symbol visibility change

Hi all,

Could you update the Changelog for this PR? boostorg/boost#190 
It enables hidden visibility by default, which is a breaking change for me.

I think it's merged since 1.69. Didn't see it mentioned in the Changelog: https://www.boost.org/doc/libs/1_71_0/libs/log/doc/html/log/changelog.html

Thanks,
Quanlong