SWVertexLoader: Make SWVertexLoader a final class

It doesn't make sense to allow extending this class based off the way
the VertexLoader API is set up.

Source/Core/VideoBackends/Software/SWVertexLoader.h

@@ -14,7 +14,7 @@
 
 #include "VideoCommon/VertexManagerBase.h"
 
-class SWVertexLoader : public VertexManagerBase
+class SWVertexLoader final : public VertexManagerBase
 {
 public:
   SWVertexLoader();