fix(Pointer): ensure required include is used for unity 5.5

When using Navmesh in Unity 5.5 it requires to be prefixed with the
UnityEngine.AI package. To prevent issues when upgrading to Unity 5.5
this is now included at the top of the class if using Unity 5.5.

Assets/VRTK/Scripts/Pointers/PointerRenderers/VRTK_BasePointerRenderer.cs

@@ -3,6 +3,9 @@ namespace VRTK
 {
     using UnityEngine;
     using System.Collections.Generic;
+#if UNITY_5_5_OR_NEWER
+    using UnityEngine.AI;
+#endif
 
     /// <summary>
     /// The Base Pointer Renderer script is an abstract class that handles the set up and operation of how a pointer renderer works.