selftests: pidfd: Add pidfd_fdinfo_test in .gitignore

The commit identified below added pidfd_fdinfo_test
but failed to add it to .gitignore

Fixes: 2def297e

 ("pidfd: add tests for NSpid info in fdinfo")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/966567c7dbaa26a06730d796354f8a086c0ee288.1582847778.git.christophe.leroy@c-s.fr
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>