Need to decorate XactIsoLevel as PGDLLIMPORT for postgres_fdw.

Per buildfarm.

contrib/postgres_fdw/connection.c

@@ -305,7 +305,7 @@ begin_remote_xact(ConnCacheEntry *entry)
 		elog(DEBUG3, "starting remote transaction on connection %p",
 			 entry->conn);
 
-		if (XactIsoLevel == XACT_SERIALIZABLE)
+		if (IsolationIsSerializable())
 			sql = "START TRANSACTION ISOLATION LEVEL SERIALIZABLE";
 		else
 			sql = "START TRANSACTION ISOLATION LEVEL REPEATABLE READ";

src/include/access/xact.h

@@ -28,7 +28,7 @@
 #define XACT_SERIALIZABLE		3
 
 extern int	DefaultXactIsoLevel;
-extern int	XactIsoLevel;
+extern PGDLLIMPORT int XactIsoLevel;
 
 /*
  * We implement three isolation levels internally.