[cleanup] Fix license checker after #11851

1. Exclude //third_party from license checks.
2. Add license headers to a few files.

Signed-off-by: Chris Frantz <cfrantz@google.com>
diff --git a/util/licence-checker.hjson b/util/licence-checker.hjson
index 4e4fe63..494da95 100644
--- a/util/licence-checker.hjson
+++ b/util/licence-checker.hjson
@@ -10,6 +10,7 @@
     ''',
   exclude_paths: [
     # Exclude anything in vendored directories
+    'third_party/**',
     '*/vendor/*/*',
     'util/lowrisc_misc-linters/*',