fix(SDK): exclude BuildTargetGroup outside Unity Editor

BuildTargetGroup is defined in `UnityEditor` namespace, exclude the
declaration of `excludeTargetGroups` in `VRTK_SDKManager` if not in
Unity Editor.

Assets/VRTK/Source/Scripts/Utilities/SDK/VRTK_SDKManager.cs

@@ -180,13 +180,15 @@ public static VRTK_SDKManager instance
  public bool autoLoadSetup = true;
  [Tooltip("The list of SDK Setups to choose from.")]
  public VRTK_SDKSetup[] setups = new VRTK_SDKSetup[0];
+#if UNITY_EDITOR
  [Tooltip("The list of Build Target Groups to exclude.")]
  public BuildTargetGroup[] excludeTargetGroups = new BuildTargetGroup[] {
 #if UNITY_2017_1_OR_NEWER
  BuildTargetGroup.Switch,
  BuildTargetGroup.Facebook
 #endif
  };
+#endif
 
  /// <summary>
  /// The loaded SDK Setup. `null` if no setup is currently loaded.
@@ -216,8 +218,9 @@ public VRTK_SDKSetup loadedSetup
  private Coroutine checkLeftControllerReadyRoutine = null;
  private Coroutine checkRightControllerReadyRoutine = null;
  private float checkControllerReadyDelay = 1f;
+#if UNITY_EDITOR
  private BuildTargetGroup[] targetGroupsToExclude;
-
+#endif
  /// <summary>
  /// The event invoked whenever the loaded SDK Setup changes.
  /// </summary>