Add const qualifier to tzn returned by timestamp2tm()

The tzn value might come from tm->tm_zone, which libc declares as
const, so it's prudent that the upper layers know about this as well.

src/backend/utils/adt/datetime.c

@@ -4337,7 +4337,7 @@ pg_timezone_names(PG_FUNCTION_ARGS)
 	int			tzoff;
 	struct pg_tm tm;
 	fsec_t		fsec;
-	char	   *tzn;
+	const char *tzn;
 	Interval   *resInterval;
 	struct pg_tm itm;