Add missing backslash to delete addressbook method

Relevant error: 
[09-Feb-2013 17:03:31] PHP Fatal error:  Class 'OCA\Contacts\OCP\User' not found in /Users/tomneedham/Sites/htdocs/cloud/apps2/contacts/lib/addressbook.php on line 349

Relevant Backtrace:
[09-Feb-2013 17:03:31] PHP   7. OC_User::deleteUser() /Users/tomneedham/Sites/htdocs/cloud/apps2/provisioning_api/lib/users.php:169
[09-Feb-2013 17:03:31] PHP   8. OC_Hook::emit() /Users/tomneedham/Sites/htdocs/cloud/lib/user.php:228
[09-Feb-2013 17:03:31] PHP   9. call_user_func() /Users/tomneedham/Sites/htdocs/cloud/lib/hook.php:71
[09-Feb-2013 17:03:31] PHP  10. OCA\Contacts\Hooks::deleteUser() /Users/tomneedham/Sites/htdocs/cloud/lib/hook.php:0
[09-Feb-2013 17:03:31] PHP  11. OCA\Contacts\Addressbook::delete() /Users/tomneedham/Sites/htdocs/cloud/apps2/contacts/lib/hooks.php:59