segments/hostname.sh: command_exists not found

This function
```
command_exists() {
       command -v $1 > /dev/null 
}
```

no longer in hostname.sh so the call to the function fails.

I assume you probably want to source util.sh as done in segments/now_playing.sh
```
source "${TMUX_POWERLINE_DIR_LIB}/util.sh"
```

adding this line does fix the issue.