selftests: pidfd: Create .gitignore to include pidfd_test

Create ../selftests/pidfd/.gitignore which holds the following file name
created after compiling:

	- pidfd_test

Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>