Add abstract method ensure_siblings to the sibling baseclass.

This should have been here for ever. At least all the implementors
already implement it.
Signed-off-by: Nigel McNie <nigel@catalyst.net.nz>

htdocs/artefact/resume/export/leap/lib.php

@@ -306,6 +306,8 @@ abstract class LeapExportElementResumeCompositeChildWithSiblings extends LeapExp
         }
         return $this->siblings;
     }
+
+    public abstract function ensure_siblings();
 }
 
 class LeapExportElementResumeCompositeChildEducationhistory extends LeapExportElementResumeCompositeChildWithSiblings {