rcutorture/nolibc: Fix a typo in header file

This fixes a typo. Before this, the AT_FDCWD macro would be defined
regardless of whether or not it's been defined before.

Signed-off-by: Samuel Hernandez <sam.hernandez.amador@gmail.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>