selftests: fchmodat2: remove duplicate unneeded defines

These duplicate defines should automatically be picked up from kernel
headers. Use KHDR_INCLUDES to add kernel header files.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Message-Id: <20230805073809.1753462-4-usama.anjum@collabora.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>