PGDLLIMPORT-ify old_snapshot_threshold.

Revert commit 7cb1db1d95, which represented
a misunderstanding of the problem (if snapmgr.h weren't already included
in bufmgr.h, things wouldn't compile anywhere).  Instead install what
I think is the real fix.

contrib/bloom/blscan.c

@@ -19,7 +19,6 @@
 #include "storage/lmgr.h"
 #include "utils/memutils.h"
 #include "utils/rel.h"
-#include "utils/snapmgr.h"
 
 #include "bloom.h"
 

src/include/utils/snapmgr.h

@@ -20,7 +20,7 @@
 
 
 /* GUC variables */
-extern int	old_snapshot_threshold;
+extern PGDLLIMPORT int old_snapshot_threshold;
 
 
 extern Size SnapMgrShmemSize(void);