lighttpd: fix build failure without ipv6

Reported in https://bugs.gentoo.org/show_bug.cgi?id=486426
and http://redmine.lighttpd.net/issues/2515


Autobuilders didn't hit it yet, but i did.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>