Set the lock object as readonly

Author: lindexi

src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/Stylus/Wisp/WispLogic.cs

@@ -4044,7 +4044,7 @@ internal long IncrementVersion()
         /// Lock the access to coalesced moves as it's possible it can be accessed simultaneously from two
         /// PenThreads if the initial PenThread fills up with PenContexts.
         /// </summary>
-        object _coalesceLock = new object();
+        private readonly object _coalesceLock = new object();
 
 #if !MULTICAPTURE
         IInputElement _stylusCapture;