crypto/x509: generate new-style build tags for iOS

Make the input match gofmt's output,
to make our lives easier as we phase out
old style build tags.

Change-Id: I95dc5a77058bf17cb02e289703f60784616db006
Reviewed-on: https://go-review.googlesource.com/c/go/+/358934
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/crypto/x509/root_ios_gen.go

@@ -164,8 +164,8 @@ func main() {
 const header = `// Code generated by root_ios_gen.go -version %s; DO NOT EDIT.
 // Update the version in root.go and regenerate with "go generate".
 
-// +build ios
-// +build !x509omitbundledroots
+//go:build ios && !x509omitbundledroots
+// +build ios,!x509omitbundledroots
 
 package x509