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_opengl/gl_rasterizer.h

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