fix: using GLPI_ROOT if defined of set to relative path

[djerfy]

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

Versions:

• GLPI: 11.0.0-rc3
• DataInjection: 2.15.0-beta2

With Docker, GLPI is installed in /var/www/glpi by default and the marketplace directory is /var/glpi/marketplace. So the includes.php requirements is broken with this relative path. This MR use GLPI_ROOT variable if defined (else it's a relative path).

Screenshots (if appropriate):