selftests: pidfd: Add pidfd_fdinfo_test in .gitignore

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

Fixes: 2def297ec7fb ("pidfd: add tests for NSpid info in fdinfo")
Signed-off-by: Christophe Leroy <[e-mail]>
Acked-by: Christian Brauner <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/966567c7[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

tools/testing/selftests/pidfd/.gitignore

···
 pidfd_poll_test
 pidfd_test
 pidfd_wait
+pidfd_fdinfo_test
 pidfd_getfd_test