Use SDL_NORETURN

I misread SDL's code and thought that SDL's `begin_code.h` was internal
only to SDL. It turns out you get it when you include basically any
header, such as `SDL_stdinc.h`. So use it directly instead of copying it
for our own.

desktop_version/src/main.cpp

@@ -708,7 +708,7 @@ static void cleanup(void)
     FILESYSTEM_deinit();
 }
 
-VVV_NORETURN void VVV_exit(const int exit_code)
+SDL_NORETURN void VVV_exit(const int exit_code)
 {
     cleanup();
     exit(exit_code);