Fixes external linking of net/http tests (or anything that uses
sendfile).

Fixes #12390.

Change-Id: Iee08998cf66e7b0ce851db138a00ebae6dc2395e
Reviewed-on: https://go-review.googlesource.com/14072Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>