Fix HTTPServer gpu-renderer interfacing

Fixing some compilation issues that occur when enabling the HTTP server

include/PICA/gpu.hpp

@@ -85,6 +85,7 @@ class GPU {
 	GPU(Memory& mem, EmulatorConfig& config);
 	void initGraphicsContext() { renderer->initGraphicsContext(); }
 	void display() { renderer->display(); }
+	void screenshot(const std::string& name) { renderer->screenshot(name); }
 
 	void fireDMA(u32 dest, u32 source, u32 size);
 	void reset();