Fix unused function warning when readline enabled (mirb)

Fix below warning occured when READLINE enabled.

```sh
CC    mrbgems/mruby-bin-mirb/tools/mirb/mirb.c -> build/host/mrbgems/mruby-bin-mirb/tools/mirb/mirb.o
/Users/koji/work/mruby/mruby/mrbgems/mruby-bin-mirb/tools/mirb/mirb.c:223:1: warning: unused function 'print_cmdline'
      [-Wunused-function]
print_cmdline(int code_block_open)
^
1 warning generated.
```