fix: Make sure that require 'config.php' is first

[edalex-ian]

Checklist

• [x] the contributor license agreement is signed
• [x] commit message follows commit guidelines
• [ ] screenshots are included showing significant UI changes
  • not applicable
• [ ] documentation is changed or added
  • not applicable

Description of change

Although on some configurations existing code works, on others it will result in attempting to create a session after a response has already started. General moodle guidance is that require_once('config.php') should be first in any files which are the result of requests dealt with the handler.php.