fix missing locallib requirement and other issues

Hi Gordon,

I encountered the following error when re entering in a course.

Error message : Fatal error: Class 'mod_taskchain' not found in ./mod/taskchain/lib.php on line 817

the problem was localized in the function: taskchain_print_recent_activity();

As other functions can be concerned I choose to add the requirement call at the top of the file.

Best regards

Eric