Fix a mismatch between declaration and definition

The `CMD_HANDLERS` macro defines `cmd_handlers` as `const`

```
#define CMD_HANDLERS(...) \
const cmd_handler_t cmd_handlers[] = {__VA_ARGS__, NULL}
```
whereas the `extern` declaration in `cmd.c` does not use `const`.

```
extern cmd_handler_t cmd_handlers[];
```

SDCC really dislikes this situation and silently treats the two `cmd_handlers` as residing in different memory spaces: one is in `CONST` and the other in `XDATA`.