fixed missing import

Author: thjaeckle

model/things/src/main/java/org/eclipse/ditto/model/things/ImmutableFeatures.java

@@ -29,6 +29,7 @@
 import org.eclipse.ditto.json.JsonFactory;
 import org.eclipse.ditto.json.JsonField;
 import org.eclipse.ditto.json.JsonFieldDefinition;
+import org.eclipse.ditto.json.JsonKey;
 import org.eclipse.ditto.json.JsonObject;
 import org.eclipse.ditto.json.JsonObjectBuilder;
 import org.eclipse.ditto.json.JsonPointer;