Added support for extra backends (see pull request #5043)

Somebody had forgotten "OC_User::setupBackends();"...

lib/private/connector/sabre/auth.php

@@ -36,6 +36,8 @@ class OC_Connector_Sabre_Auth extends Sabre_DAV_Auth_Backend_AbstractBasic {
 			return true;
 		} else {
 			OC_Util::setUpFS();//login hooks may need early access to the filesystem
+			//setup extra user backends
+			OC_User::setupBackends();//if you do not want to use Owncloud's integrated login
 			if(OC_User::login($username, $password)) {
 				OC_Util::setUpFS(OC_User::getUser());
 				return true;