Add missing namespace

Author: keveleigh

org.mixedrealitytoolkit.spatialmanipulation/Tests/Runtime/ObjectManipulatorTests.cs

@@ -14,6 +14,7 @@
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.TestTools;
+using UnityEngine.XR.Interaction.Toolkit.Interactables;
 using UnityEngine.XR.Interaction.Toolkit.Interactors;
 
 using HandshapeId = MixedReality.Toolkit.Input.HandshapeTypes.HandshapeId;