fix compile error

As auto-type-cast is not procedured for some reason, procedure type-cast explicitly.

```
CC    src/jitcode.cc -> build/host/src/jitcode.o
/home/bokko/workspace/mruby/mruby/src/jitcode.cc: In function ‘const void* mrbjit_emit_code(mrb_state*, mrbjit_vmstatus*, mrbjit_code_info*)’:
/home/bokko/workspace/mruby/mruby/src/jitcode.cc:254:36: error: cannot pass objects of non-trivially-copyable type ‘class Xbyak::Error’ through ‘...’
/home/bokko/workspace/mruby/mruby/src/jitcode.cc:254:36: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘Xbyak::Error’ [-Wformat]
```

## my environment

 * GCC-4.7.3
 * Linux Mint 15(kernel-3.8.0-31-generic amd64)