dhcp: fix parsing warning

* fixes:
  WARNING: Variable key FILES_${PN}-client (${base_sbindir}/dhclient ${base_sbindir}/dhclient-script ${sysconfdir}/dhcp/dhclient.conf)
    replaces original key FILES_dhcp-client ( ${systemd_unitdir}/system/dhclient.service).

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>