Extension autoload.php is only included when compiling container

The autoload.php for extensions is no longer included in every request in 3.2, effectively breaking any that make use of it. Whereas in 3.1 this was done in common.php using the phpbb_load_extensions_autoloaders function, in 3.2 this now takes place when loading extensions to compile the container. https://github.com/phpbb/phpbb/blob/master/phpBB/phpbb/di/container_builder.php#L381-L386