Bug fix: Callstack debug logging in .NET assembly was broken

By cc3eaac3

Source commit: 3e2f1bfae157d5ccc4209891f5421bf8e24c4bd0

dotnet/Session.cs

@@ -124,7 +124,7 @@ namespace WinSCP
             }
         }
 
-        private IDisposable CreateCallstackAndLock()
+        private CallstackAndLock CreateCallstackAndLock()
         {
             var result = Logger.CreateCallstackAndLock();
             if ((_process != null) && (_process.StdOut != null))