Fix decoding of renameat2 RENAME_* flags

* renameat.c: Include <linux/fs.h> where RENAME_NOREPLACE,
RENAME_EXCHANGE, and RENAME_WHITEOUT are usually defined.
diff --git a/renameat.c b/renameat.c
index bf3dd79..1c02551 100644
--- a/renameat.c
+++ b/renameat.c
@@ -19,6 +19,7 @@
 	return 0;
 }
 
+#include <linux/fs.h>
 #include "xlat/rename_flags.h"
 
 int