selftests/filesystems: devpts_pts included wrong header

We were picking up the wrong header should use asm/ioctls.h form the kernel
and not the header from the system (sys/ioctl.h). In the current code we
added the correct include and we added the kernel headers path to the CFLAGS.

Fixes: ce290a19 ("selftests: add devpts selftests")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>