jobs: realm sync: fix scheduled LDAP syncs not applying attributes correctly

jobs: realm sync: fix scheduled LDAP syncs not applying attributes correctly

This was reported by a user in the forum [0].

The cause was that the user-* standard options were not registered
when the sync was called from the scheduler, resulting in the
following error:

  pvescheduler[2849]: skipping attribute mapping 'cn'->'comment' for user 'test@samba0' - no such standard option 'user-comment'

Fix this by simply importing the PVE::API2::User module, thus ensuring
the options get registered.

[0] https://forum.proxmox.com/threads/ldap-integration-comment-email-first-name-lastname.143490/

Fixes: cb93636 ("LDAP sync: improve validation of synced attributes")
Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
Tested-by: Fiona Ebner <f.ebner@proxmox.com>