add child3 to updated list

Child3 has its mesh filter updated even though it wasn't directly
modified. This is because the FileID in the meta file changed. For
example:

oldvalue :
{"MeshFilter":{"m_Mesh":{"fileID":4300008,"guid":"9e8f66f421db4884f8bfe401c8c4947f","type":3}}}
new value:
{"MeshFilter":{"m_Mesh":{"fileID":4300002,"guid":"9e8f66f421db4884f8bfe401c8c4947f","type":3}}}

Author: vkovec

Assets/FbxExporters/Editor/UnitTests/FbxPrefabTest.cs

@@ -251,7 +251,7 @@ public void BasicTest() {
                 // transform changed, Parent3 was created.
                 Assert.AreEqual (1, updateSet.NumUpdates);
                 Assert.That (updateSet.Updated, Is.EquivalentTo (new string [] {
-                    "Parent2", "Parent3"
+                    "Parent2", "Parent3", "Child3"
                 }
                 ));