huexpress: add workaround for newer clang and gcc 10+

[chenrui333]

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  In file included from src/engine/h6280.c:8:
  src/engine/dis_runtime.h:544:3: error: incompatible function pointer types initializing 'int (*)(void)' with an expression of type 'void (void)' [-Wincompatible-function-pointer-types]
    544 |         {handle_bios, AM_IMPL, "???"}
        |          ^~~~~~~~~~~
  1 error generated.
  scons: *** [src/engine/h6280.o] Error 1

• incompatible integer to pointer conversion build errors kallisti5/huexpress#19
• error when compiling - redefinition of xBuf kallisti5/huexpress#18