Fix namespace references in AnnotatedMessage

PHP can't find the namespaces properly because the class and namespace are identical.

```php
use DrSlump\Protobuf;
```

Fixes
```
PHP Fatal error:  Class 'DrSlump\\Protobuf' not found in /var/www/test-proto/vendor/centraldesktop/protobuf-php/library/DrSlump/Protobuf/AnnotatedMessage.php on line 77
```