sdl/video: Add placeholder SDL_FlashOperation enum when using SDL2 older than 2.0.16
sdl/video: Add placeholder SDL_FlashOperation enum when using SDL2 older than 2.0.16

Signed-off-by: Lilis Iskandar <f9acb790f2c75b127492316321ece0121b8eed9b@veand.co>

veandco/go-sdl2,veandco/go-sdl2