Make storage/standby.h compile standalone again.

This file has failed headerscheck/cpluspluscheck verification since
commit 0650ff23, as a result of referencing typedef TimestampTz
without including the appropriate header.

src/include/storage/standby.h

@@ -14,6 +14,7 @@
 #ifndef STANDBY_H
 #define STANDBY_H
 
+#include "datatype/timestamp.h"
 #include "storage/lock.h"
 #include "storage/procsignal.h"
 #include "storage/relfilenode.h"