add SDL3 close code include

[alxyng]

Fixes the nested inclusion error

error: Nested inclusion of SDL_begin_code.h

when including SDL library headers after this one e.g.

#define SDL_MAIN_USE_CALLBACKS 1
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>
#include <SDL3_gpu_shadercross/SDL_gpu_shadercross.h>
#include <SDL3_image/SDL_image.h>

[flibitijibibo]

I didn't realize we were including begin_code... lgtm but if we can remove begin/end that'd be even better.

[alxyng]

Guess it's a style thing. Looks like SDL_begin_code.h defines SDL_DECLSPEC which the header uses but the macro can be depended on transitively through SDL3/SDL.h and friends. Up to you guys.