Avoid fatal PHP error due to deprecation-warnings during autoloading.

Due to a PHP bug combined with deprecation-warnings from the OpenID
library, we may fail with a fatal error due to not finding the
SimpleSAML_Logger class.

See PHP bug: https://bugs.php.net/bug.php?id=47987

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2999 44740490-163a-0410-bde0-09ae8108e29a