declare struct mrb_irep.

When ENABLE_DEBUG is defined, compiler warns that struct mrb_irep is not properly declared:

```
CC    tools/mrbc/mrbc.c -> build/host/tools/mrbc/mrbc.o
In file included from /Users/tsahara/Projects/mruby/tools/mrbc/mrbc.c:4:
/Users/tsahara/Projects/mruby/include/mruby.h:152:57: warning: declaration of 'struct mrb_irep' will not be visible outside of this function [-Wvisibility]
  void (*code_fetch_hook)(struct mrb_state* mrb, struct mrb_irep *irep, mrb_code *pc, mrb_value *regs);
                                                        ^
1 warning generated.
```