license headers: add dnsname authors to copyright

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

plugins/meta/dnsname/dnsname_suite_test.go

+// Copyright 2019 dnsname authors
 // Copyright 2017 CNI authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");

plugins/meta/dnsname/main.go

+// Copyright 2019 dnsname authors
 // Copyright 2017 CNI authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");