core.php: move config_inc.php include earlier in the file

This fixes a regression introduced by the new config folder commit
4b173463c2715c7dfa12dc37b6663df19930e594: global variables overriden in
config_inc.php are set to their default value in the config cache,
causing config_get_global() to return an incorrect value.

Moving the code to include the config_inc.php file back to its previous
location (immediately after config_defaults_inc.php and before
initializing the various APIs) fixes the problem.

Fixes #17263