Update app.hpp

app.hpp

 #ifndef APP_HPP
 #define APP_HPP
-#include <string>
 #include <vector>
+#include "chess.hpp"
 
 class Widget;
 
@@ -15,7 +15,7 @@ public:
     virtual ~App();
 
     void register_widget(Widget*);
-    void event_loop();
+    void event_loop(Chess*);
 };
 
 #endif // APP_HPP