There's no header file associated with main.cpp because its only purpose is to act as an entry point to the application. In fact, the file is only seven lines long:

#include "game.h"

int main() {
    Game game;
    while (game.loop());
    return 0;
}

The while statement is exited when the loop() function returns false, which occurs when the SDL_QUIT event fires. All this file is doing is creating a new instance of Game and starting the loop. That's it for the codebase; let's start porting!