support 2018.1 and 2018.2 api changes

Author: inwoods

Assets/com.unity.formats.fbx/Editor/Scripts/FbxExporter.cs

@@ -1289,7 +1289,11 @@ protected bool ExportInstance (GameObject unityGo, FbxNode fbxNode, FbxScene fbx
                 if (unityPrefabType != PrefabType.PrefabInstance &&
                     unityPrefabType != PrefabType.ModelPrefabInstance) return false;
 
+                #if UNITY_2018_2_OR_NEWER
                 Object unityPrefabParent = PrefabUtility.GetCorrespondingObjectFromSource (unityGo);
+                #else
+                Object unityPrefabParent = PrefabUtility.GetPrefabParent (unityGo);
+                #endif
 
                 if (Verbose)
                     Debug.Log (string.Format ("exporting instance {0}({1})", unityGo.name, unityPrefabParent.name));