Fix SPDX license header (#231)

This change adds SPDX license headers to all files and eforces it with
the linter.

Signed-off-by: Philip Laine <philip.laine@gmail.com>

.github/workflows/go.yaml

@@ -31,7 +31,7 @@ jobs:
           git diff --exit-code
       - name: Verify generate is not dirty
         run: |
-          make generate
+          make -B generate
           git diff --exit-code
       - name: Running unit tests
         run: make test