treewide: add license header and enable haslicense linter

Change-Id: I873a8d4082d75e8f813d8a726a41187eea7a065e
Reviewed-on: https://review.monogon.dev/c/monogon/+/3825
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/osbase/gpt/gpt.go b/osbase/gpt/gpt.go
index 126021b..3d3cadb 100644
--- a/osbase/gpt/gpt.go
+++ b/osbase/gpt/gpt.go
@@ -1,3 +1,6 @@
+// Copyright The Monogon Project Authors.
+// SPDX-License-Identifier: Apache-2.0
+
 // Package gpt implements reading and writing GUID Partition Tables as specified
 // in the UEFI Specification. It only implements up to 128 partitions per table
 // (same as most other implementations) as more would require a dynamic table