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>
diff --git a/src/net/http/h2_bundle.go b/src/net/http/h2_bundle.go
index df98232..29226d4 100644
--- a/src/net/http/h2_bundle.go
+++ b/src/net/http/h2_bundle.go
@@ -1,4 +1,5 @@
 //go:build !nethttpomithttp2
+// +build !nethttpomithttp2
 
 // Code generated by golang.org/x/tools/cmd/bundle. DO NOT EDIT.
 //   $ bundle -o=h2_bundle.go -prefix=http2 -tags=!nethttpomithttp2 golang.org/x/net/http2