tcp: fix alternate stack build with LINT-NO{INET,INET6,IP}

When fixing another bug, I noticed that the alternate
TCP stacks do not build when various combinations of
ipv4 and ipv6 are disabled.

Reviewed by: rrs, tuexen
Differential Revision: https://reviews.freebsd.org/D31094
Sponsored by: Netflix

(cherry picked from commit b1e806c0ed960e1eb9ee889c7d0df3c168290c4f)