testing/vsock: add vsock_perf to gitignore

This adds the vsock_perf binary to the gitignore file.

Fixes: 8abbffd2 ("test/vsock: vsock_perf utility")
Signed-off-by: Bobby Eshleman <bobby.eshleman@bytedance.com>
Reviewed-by: Arseniy Krasnov <AVKrasnov@sberdevices.ru>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20230327-vsock-add-vsock-perf-to-ignore-v1-1-f28a84f3606b@bytedance.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>