Kernel: Standardize the header include style to 'include <Kernel/...>'

This is the overwhelming standard in the project, but there were some
cases in the kernel which were not following it, lets fix those cases!

Kernel/ProcessGroup.cpp

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#include "ProcessGroup.h"
+#include <Kernel/ProcessGroup.h>
 
 namespace Kernel {