fb_generic.h: Add missing function add_gxa_sync_marker()

- is required for cs hd1

src/driver/fb_generic.h

@@ -185,6 +185,7 @@ class CFrameBuffer : public sigc::trackable
 		bool getActive() const;                     // is framebuffer active?
 		void setActive(bool enable);                     // is framebuffer active?
 		virtual void setupGXA() { return; };             // reinitialize stuff
+		virtual void add_gxa_sync_marker() { return; };
 		virtual bool needAlign4Blit() { return false; };
 		virtual uint32_t getWidth4FB_HW_ACC(const uint32_t x, const uint32_t w, const bool max=true);