Blind attempt at fixing Win32 issue on 24c5f1a103

As best as I can tell, MyReplicationSlot needs to be PGDLLIMPORT in
order for the new test_slot_timelines test module to compile.

Per buildfarm

src/include/replication/slot.h

@@ -144,7 +144,7 @@ typedef struct ReplicationSlotCtlData
  * Pointers to shared memory
  */
 extern ReplicationSlotCtlData *ReplicationSlotCtl;
-extern ReplicationSlot *MyReplicationSlot;
+extern PGDLLIMPORT ReplicationSlot *MyReplicationSlot;
 
 /* GUCs */
 extern PGDLLIMPORT int max_replication_slots;