Fix missing S_IRUSR and S_IWUSR constants.

Index: src/backend/distributed/commands/multi_copy.c
--- src/backend/distributed/commands/multi_copy.c.orig
+++ src/backend/distributed/commands/multi_copy.c
@@ -55,6 +55,7 @@
 #include <arpa/inet.h> /* for htons */
 #include <netinet/in.h> /* for htons */
 #include <string.h>
+#include <sys/stat.h>
 
 #include "distributed/pg_version_constants.h"
 
