Fixed mapping entities

There were this error in the console:

> The association Wallabag\CoreBundle\Entity\SiteCredential#user refers to the inverse side field Wallabag\UserBundle\Entity\User#site_credentials which does not exist.

src/Wallabag/UserBundle/Entity/User.php

@@ -92,6 +92,13 @@ class User extends BaseUser implements TwoFactorInterface, TrustedComputerInterf
      */
     protected $config;
 
+    /**
+     * @var ArrayCollection
+     *
+     * @ORM\OneToMany(targetEntity="Wallabag\CoreBundle\Entity\SiteCredential", mappedBy="user", cascade={"remove"})
+     */
+    protected $site_credentials;
+
     /**
      * @var ArrayCollection
      *