Add missing PGDLLIMPORT markings

Since commit 8ec5694, we have a policy of marking all backend
variables with PGDLLIMPORT.

Reported-by: Anton A. Melnikov
Discussion: https://www.postgresql.org/message-id/0b78546c-ffef-4cd9-9ba1-d1e6aab88cea@postgrespro.ru

Author: hlinnaka

src/include/postmaster/walsummarizer.h

@@ -16,8 +16,8 @@
 
 #include "access/xlogdefs.h"
 
-extern bool summarize_wal;
-extern int	wal_summary_keep_time;
+extern PGDLLIMPORT bool summarize_wal;
+extern PGDLLIMPORT int wal_summary_keep_time;
 
 extern Size WalSummarizerShmemSize(void);
 extern void WalSummarizerShmemInit(void);

src/include/storage/buf_internals.h

@@ -385,8 +385,8 @@ typedef struct CkptSortItem
 extern PGDLLIMPORT CkptSortItem *CkptBufferIds;
 
 /* ResourceOwner callbacks to hold buffer I/Os and pins */
-extern const ResourceOwnerDesc buffer_io_resowner_desc;
-extern const ResourceOwnerDesc buffer_pin_resowner_desc;
+extern PGDLLIMPORT const ResourceOwnerDesc buffer_io_resowner_desc;
+extern PGDLLIMPORT const ResourceOwnerDesc buffer_pin_resowner_desc;
 
 /* Convenience wrappers over ResourceOwnerRemember/Forget */
 static inline void