print_recent_activity() assums activity

If an activity module exists in the database, but it's files have been removed, print_recent_activity() (in course/lib.php) will try to include it anyways without checking if it exists. This produces an error message.

The include_once statement is on line 1125 for the current 1.8 stable version (1.484.2.27). It should check if the file exists first like it does in line 975.