[ticket/12871] Add PHPBB_DISPLAY_LOAD_TIME const to config.php on installation

To make it consistent with another config.php constants,
PHPBB_DISPLAY_LOAD_TIME is to be put in config.php during installation as well.

PHPBB3-12871
Merge pull request #2757 from rxu/ticket/12871

[ticket/12871] Add PHPBB_DISPLAY_LOAD_TIME const to config.php on installation

* rxu/ticket/12871:
  [ticket/12871] Add PHPBB_DISPLAY_LOAD_TIME const to config.php on installation