fix(SDK): ensure object references are populated on enable

There was an issue where the SDK Setup object references were not
being auto populated on scene change which was causing certain
scripts to not be parented correctly.

This fix ensures the references are populated when the SDK Setup
script is enabled.

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

@@ -395,6 +395,11 @@ public void OnUnloaded(VRTK_SDKManager sender)
             }
         }
 
+        private void OnEnable()
+        {
+            PopulateObjectReferences(false);
+        }
+
 #if UNITY_EDITOR
         static VRTK_SDKSetup()
         {