[ticket/12018] Use path_helper for admin style CSS in sql report

The path to the admin style CSS is currently created with $phpbb_admin_path.
We should however use the path_helper to correctly link to this file in order
to have a correct link on pages like extensions

PHPBB3-12018
Merge remote-tracking branch 'Marc/ticket/12018' into develop

* Marc/ticket/12018:
  [ticket/12018] Use path_helper for admin style CSS in sql report