Fix parsing CycloneDX 1.6 declarations/definitions elements

[pasieronen]

For example, this SBOM

{
  "bomFormat": "CycloneDX",
  "specVersion": "1.6",
  "components": [
    {
      "type": "library",
      "name": "somelib"
    }
  ],
  "definitions": {
    "standards": []
  }
}

passes sbom-utility validate, but if e.g. sbom-utility component fails with error Error: json: cannot unmarshal object into Go struct field CDXBom.definitions of type []schema.CDXDefinition.