video_core: Amend misplaced forward declarations

ShaderProgramManager was placed within the wrong namespace. Backend
simply isn't necessary, so it can be removed.

src/video_core/renderer_base.h

@@ -13,10 +13,6 @@ namespace Frontend {
 class EmuWindow;
 }
 
-namespace FrameDumper {
-class Backend;
-}
-
 class RendererBase : NonCopyable {
 public:
     explicit RendererBase(Frontend::EmuWindow& window);

src/video_core/renderer_opengl/gl_rasterizer.h

@@ -33,9 +33,8 @@ namespace Frontend {
 class EmuWindow;
 }
 
-class ShaderProgramManager;
-
 namespace OpenGL {
+class ShaderProgramManager;
 
 class RasterizerOpenGL : public VideoCore::RasterizerInterface {
 public: