Fix fatal private method error

When trying to read data from a FLAC file, I'm receiving the following error message:
```
Fatal error:  Call to private method GetId3\GetId3Core::warning() from context 'GetId3\Handler\BaseHandler' in /vagrant/vendor/phansys/getid3/GetId3/Handler/BaseHandler.php on line 215
```
Obviously, making it public fixes the error. Unsure as to whether or not that is the best solution tho.