'PGDLLIMPORT' ShmemVariableCache, needed for pg_migrator.so function

linkage on Win32.

Tested by Hiroshi Saito

src/include/access/transam.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/access/transam.h,v 1.67 2009/01/01 17:23:56 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/access/transam.h,v 1.68 2009/05/08 03:21:35 momjian Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -130,7 +130,7 @@ typedef VariableCacheData *VariableCache;
  */
 
 /* in transam/varsup.c */
-extern VariableCache ShmemVariableCache;
+extern PGDLLIMPORT VariableCache ShmemVariableCache;
 
 
 /*