libvirtd: add missing dependency on dnsmasq

Fixes this:

Nov 09 16:13:27 nixos-laptop systemd[1]: Starting Libvirt Virtual Machine Management Daemon...
Nov 09 16:13:28 nixos-laptop .libvirtd-wrapp[17183]: libvirt version: 1.1.4
Nov 09 16:13:28 nixos-laptop .libvirtd-wrapp[17183]: Cannot check dnsmasq binary dnsmasq: No such file or directory
Nov 09 16:13:28 nixos-laptop .libvirtd-wrapp[17183]: Cannot check dnsmasq binary dnsmasq: No such file or directory
Nov 09 16:13:28 nixos-laptop systemd[1]: Started Libvirt Virtual Machine Management Daemon.