Remove compile error when SteamVR plugin not installed

Author: lawwong

Assets/HTC.UnityPlugin/ViveInputUtility/Scripts/Misc/ExternalCameraHook.cs

@@ -22,6 +22,7 @@ public class ExternalCameraHook : SingletonBehaviour<ExternalCameraHook>, INewPo
 
     private bool m_quadViewSwitch = false;
     private bool m_configInterfaceSwitch = true;
+    private GameObject m_configUI = null;
 
     public ViveRoleProperty viveRole { get { return m_viveRole; } }
 
@@ -103,8 +104,6 @@ private void OnValidate()
     private SteamVR_ExternalCamera m_externalCamera;
     private Pose m_staticExCamPose = Pose.identity;
 
-    private GameObject m_configUI;
-
     public string configPath
     {
         get