fix gmf layertree example (add missing require)
fix gmf layertree example (add missing require)

pgiraud/ngeo,camptocamp/ngeo,Jenselme/ngeo,ger-benjamin/ngeo,pgiraud/ngeo,pgiraud/ngeo,camptocamp/ngeo,camptocamp/ngeo,adube/ngeo,Jenselme/ngeo,Jenselme/ngeo,adube/ngeo,kalbermattenm/ngeo,adube/ngeo,camptocamp/ngeo,Jenselme/ngeo,kalbermattenm/ngeo,ger-benjamin/ngeo,kalbermattenm/ngeo,ger-benjamin/ngeo,adube/ngeo,camptocamp/ngeo