fix(SDKSetup): changes because of auto populate not serializing

Whenever the auto populate action run the changes done were not
saved. The fix is to let Unity know about the changes so it will
mark the scene as dirty.

This fixes #1187.

Author: Christopher - Marcel Böddecker

Assets/VRTK/Scripts/Utilities/SDK/VRTK_SDKSetup.cs

@@ -278,6 +278,10 @@ public void PopulateObjectReferences(bool force)
 #endif
             VRTK_SDK_Bridge.InvalidateCaches();
 
+#if UNITY_EDITOR
+            Undo.RecordObject(this, "Populate Object References");
+#endif
+
             actualBoundaries = null;
             actualHeadset = null;
             actualLeftController = null;