Fix build failure on systems missing IPV6_TCLASS
Fix build failure on systems missing IPV6_TCLASS

Some operating systems such as GNU/Hurd support IPv6 but do not define
the IPV6_TCLASS socket option.

Signed-off-by: Mike Miller <cbf758dc7f01d8e2fb9312684cb62ccbff81fc24@debian.org>
Signed-off-by: David Woodhouse <97b3379caa91f4ee97e44013ae4dc6350540fa9d@infradead.org>

brandt/openconnect,brandt/openconnect,brandt/openconnect,brandt/openconnect