Add constant export declaration for MRBC output compiled as C++

Otherwise, C++ compilers will skip this constant when producing object files.

src/dump.c

@@ -1059,13 +1059,15 @@ mrb_dump_irep_cfunc(mrb_state *mrb, mrb_irep *irep, uint8_t flags, FILE *fp, con
       return MRB_DUMP_WRITE_FAULT;
     }
     if (fprintf(fp,
+          "extern const uint8_t %s[];\n"
           "const uint8_t\n"
           "#if defined __GNUC__\n"
           "__attribute__((aligned(%u)))\n"
           "#elif defined _MSC_VER\n"
           "__declspec(align(%u))\n"
           "#endif\n"
           "%s[] = {",
+          initname,
           (uint16_t)MRB_DUMP_ALIGNMENT, (uint16_t)MRB_DUMP_ALIGNMENT, initname) < 0) {
       mrb_free(mrb, bin);
       return MRB_DUMP_WRITE_FAULT;