hosts/gentoo: fix nonstandard path to systemctl command

Starting Vagrant on my Gentoo machine with systemd outputs a "command not found" message:

```
[default] Exporting NFS shared folders...
[default] Preparing to edit /etc/exports. Administrator privileges will be required...
Password: 
sudo: /usr/sbin/systemctl: command not found
[default] Mounting NFS shared folders...
[default] Running provisioner: shell...
```

That's because /usr/sbin/systemctl is not a standard path to the systemd command in Gentoo. 
The patch provided fixes this by using the /usr/bin/systemctl path instead.