Add missing pragma once

Author: amaitland

CefSharp.BrowserSubprocess.Core/CefTaskScheduler.cpp

@@ -2,6 +2,8 @@
 //
 // Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
 
+#pragma once
+
 #include "Stdafx.h"
 #include "CefTaskScheduler.h"