v2/headers: netinet/tcp.h is missing

[kortschak]

I'm running into a ccgo compilation failure that is due to an absence of netinet/tcp.h absence. Adding the header file allows ccgo to complete, but when I add it the diff noise is huge (probably due to distro differences - I am running an ubuntu install). So rather than sending a PR, this is being filed. I can send the PR for linux_arm though if that's helpful.

[cznic]

FTR, this is now https://github.com/cznic/cc/pull/108.