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

Fixes #6864.

include/SDL3/SDL_error.h

@@ -30,7 +30,7 @@
 
 #include <SDL3/SDL_stdinc.h>
 
-#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" {
@@ -156,7 +156,7 @@ extern DECLSPEC int SDLCALL SDL_Error(SDL_errorcode code);
 #ifdef __cplusplus
 }
 #endif
-#include <SDL3/close_code.h>
+#include <SDL3/SDL_close_code.h>
 
 #endif /* SDL_error_h_ */