Give xrMemory_POOL.h pragma once, and make it more standalone.

Author: tamlin-mike

src/xrCore/xrMemory_POOL.h

@@ -1,6 +1,8 @@
+#pragma once
 #ifndef xrMemory_POOLh
 #define xrMemory_POOLh
-#pragma once
+#include "Threading/Lock.hpp"
+#include "Common/inlining_macros.h"
 
 class xrMemory;