x509roots: use "generate" build tag

Since go generate sets it automatically.

Change-Id: I4623e523392140c0472b250ac99c8c3fa31e5b15
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/504595
Auto-Submit: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>

Author: rolandshoemaker

x509roots/gen_fallback_bundle.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build gen
-// +build gen
+//go:build generate
+// +build generate
 
 //go:generate go run gen_fallback_bundle.go