fix(SDK): provide grip axis for oculus touch on steamvr

The Oculus Touch controller has a grip axis which was not being
returned via SteamVR, because the HTC Vive wand does not have an
analog grip button. However, SteamVR still returns axis information
for the Oculus Touch grip.

This grip axis is now returned correctly for SteamVR.

Assets/VRTK/SDK/SteamVR/SDK_SteamVRController.cs

@@ -463,6 +463,8 @@ public override Vector2 GetButtonAxis(ButtonTypes buttonType, VRTK_ControllerRef
                     return device.GetAxis();
                 case ButtonTypes.Trigger:
                     return device.GetAxis(EVRButtonId.k_EButton_SteamVR_Trigger);
+                case ButtonTypes.Grip:
+                    return device.GetAxis(EVRButtonId.k_EButton_Axis2);
             }
             return Vector2.zero;
         }