kbuild: fix warning when domainname is not available

Otherwise we get:
"dnsdomainname: Unknown host"

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>