sys/capability.h will be removed before FreeBSD 13. The (now) correct header to use on all supported (and most unsupported) versions of FreeBSD is sys/capsicum.h.
There's no need for autoconf checks etc. to check for both (in the ports tree) but upstream may want to if there's a desire to build on older releases.
Upstream pull request at https://github.com/golang/go/pull/28742
Port can probably use REINPLACE_CMD to modify the file while waiting on the change to percolate through, or just wait if it gets merged/released quickly.
Canonical change URL: https://go-review.googlesource.com/c/go/+/148836
Link to commit that addresses this.
This was fixed by the upstream  and included in go1.12