Add missing include guard for screen/pixel.hpp

[LiAuTraver]

I'm not sure whether it's intended or just oversighted, but the header pixel.hpp does not have include guard, for it's only included in image.hpp so there hadn't had any issue while using. I just added include guard:

#ifndef FTXUI_SCREEN_PIXEL_HPP
#define FTXUI_SCREEN_PIXEL_HPP
...
#endif // FTXUI_SCREEN_PIXEL_HPP

[ArthurSonzogni]

Thank you very much!