Mark a few logical decoding related variables with PGDLLIMPORT.

Commit 7259736a 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

src/include/access/xact.h

@@ -82,8 +82,8 @@ typedef enum
 extern int	synchronous_commit;
 
 /* used during logical streaming of a transaction */
-extern TransactionId CheckXidAlive;
-extern bool bsysscan;
+extern PGDLLIMPORT TransactionId CheckXidAlive;
+extern PGDLLIMPORT bool bsysscan;
 
 /*
  * Miscellaneous flag bits to record events which occur on the top level