Changed lingering header include guards to pragma once.

Some headers where using #ifndef to guard being including multiple times. But most were using pragma once. So for consistency I changed them all to use pragma once.

Source/Core/VideoCommon/VertexLoader_Position.h

@@ -2,8 +2,7 @@
 // Licensed under GPLv2
 // Refer to the license.txt file included.
 
-#ifndef VERTEXLOADER_POSITION_H
-#define VERTEXLOADER_POSITION_H
+#pragma once
 
 class VertexLoader_Position {
 public:
@@ -18,4 +17,3 @@ public:
 	static TPipelineFunction GetFunction(unsigned int _type, unsigned int _format, unsigned int _elements);
 };
 
-#endif