avoid the extern keyword in .cpp files

Source/Core/VideoBackends/D3D/VertexManager.cpp

@@ -16,12 +16,10 @@
 #include "VideoCommon/RenderBase.h"
 #include "VideoCommon/Statistics.h"
 #include "VideoCommon/TextureCacheBase.h"
+#include "VideoCommon/VertexLoader.h"
 #include "VideoCommon/VertexShaderManager.h"
 #include "VideoCommon/VideoConfig.h"
 
-// internal state for loading vertices
-extern NativeVertexFormat *g_nativeVertexFmt;
-
 namespace DX11
 {