The previous way was causing init blocks to be generated which the
linker wasn't smart enough to discard, and the cmd/go tool was once
again including http1 & http2 Servers.

Change-Id: I9d82e14421e0faa96437668f9013d1174f009936
Reviewed-on: https://go-review.googlesource.com/32417Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>