Mark a few logical decoding related variables with PGDLLIMPORT.
Commit
7259736a6e added two variables CheckXidAlive and bsysscan to detect
concurrent aborts and used these in inline functions that are part of the
API that can be used by extensions. So it is better to mark them with
PGDLLIMPORT.
Reported-by: Thomas Munro
Discussion: https://postgr.es/m/CA+hUKGJ7+HYupd=Pz9+QrXa-C_YnbC4rAYu8V+=OKg=UgdzMeg@mail.gmail.com