Support WebAuthN plugin v2.5.1+

[dd32]

sjinks/wp-two-factor-provider-webauthn@c0198c8

Admin::is_webauthn_enabled() no longer returns truthful on 2.5.3, as the provider class isn't present within the provider list.

I'm not sure why, but in the quick testing:

var_dump(  \Two_Factor_Core::get_providers() );
array (size=2)
  'Two_Factor_Totp' => 
    object(WordPressdotorg\Two_Factor\Encrypted_Totp_Provider)[1389]
  'Two_Factor_Backup_Codes' => 
    object(Two_Factor_Backup_Codes)[1391]

This causes the 2FA interface to be broken as the XHR endpoints for the plugin aren't loaded.