Apple in their Human Interface Guidelines asks developers to avoid using custom Face ID and Touch ID icons:
Don’t use custom icons to identify system authentication features. When people see icons that look like the system’s Touch ID (thumbprint) and Face ID icons, they think they’re supposed to authenticate. Custom variants of these icons create inconsistency and cause confusion, especially when colorized, displayed at a large size, and presented out of context—like as a button label or on an app’s Settings screen.
However they don’t provide such standard assets.
I’m not a designer but it took me few minutes to recreate the Face ID logo.
You can download the Sketch file and png, svg and pdf versions from the link below.
Update: Apple published a new page dedicated to iPhone X support that clarify their position.
Verify your Touch ID code works for Face ID. If your app currently uses Touch ID, update text strings to refer to Face ID when running on iPhone X – for example, “Sign in with Face ID.”
Avoid using icons to represent Face ID and Touch ID.
So you should not use an icon to le users login with Touch ID or Face ID, but prefer a text button.