Add missing activity level subnet field to backup.

backup/moodle2/backup_attendance_stepslib.php

@@ -44,7 +44,7 @@ class backup_attendance_activity_structure_step extends backup_activity_structur
 
         // XML nodes declaration - non-user data.
         $attendance = new backup_nested_element('attendance', array('id'), array(
-            'name', 'intro', 'introformat', 'grade', 'showsessiondetails', 'sessiondetailspos'));
+            'name', 'intro', 'introformat', 'grade', 'showsessiondetails', 'sessiondetailspos', 'subnet'));
 
         $statuses = new backup_nested_element('statuses');
         $status  = new backup_nested_element('status', array('id'), array(