kmscompositemixer: Replace kmsgenericstructure by kmsrefstruct
kmscompositemixer: Replace kmsgenericstructure by kmsrefstruct

Change-Id: I1deefdbcbfe7b6c615624eaaa6b8604f1047e69d

Kurento/kms-elements,Kurento/kms-elements,Kurento/kms-elements