Older Linux versions do not have O_NOATIME

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/os/os-linux.h b/os/os-linux.h
index 72950c0..791ec7a 100644
--- a/os/os-linux.h
+++ b/os/os-linux.h
@@ -230,6 +230,10 @@
 	return 0;
 }
 
+#ifdef O_NOATIME
 #define FIO_O_NOATIME	O_NOATIME
+#else
+#define FIO_O_NOATIME	0
+#endif
 
 #endif