Add cstdio include to flashcode

Include `<cstdio>` in the GD32 flashcode implementation so standard C stdio declarations are available where needed.

Author: vanvught

lib-flashcode/src/gd32/flashcode.cpp

@@ -25,6 +25,7 @@
 
  #include <cstdint>
  #include <cassert>
+ #include <cstdio>
  #include <span>
 
  #include "flashcode.h"