net/http: restore generated // +build comment

net/http: restore generated // +build comment

The upstream cmd/bundle tool does not yet omit it,
and the longtest builders test that the generated
file is not modified.

For #41184.

Change-Id: Ib68f532139ed1436cbaf3a756f300fe60f520cab
Reviewed-on: https://go-review.googlesource.com/c/go/+/359484
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>