include: Renamed begin_code.h and close_code.h to have SDL_ prefixes.

Fixes #6864.

include/SDL3/SDL_platform.h

@@ -207,7 +207,7 @@
 #define __3DS__ 1
 #endif
 
-#include <SDL3/begin_code.h>
+#include <SDL3/SDL_begin_code.h>
 /* Set up for C function definitions, even when using C++ */
 #ifdef __cplusplus
 extern "C" {
@@ -235,7 +235,7 @@ extern DECLSPEC const char * SDLCALL SDL_GetPlatform (void);
 #ifdef __cplusplus
 }
 #endif
-#include <SDL3/close_code.h>
+#include <SDL3/SDL_close_code.h>
 
 #endif /* SDL_platform_h_ */