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>

pkg/applyconfigurations/internal/internal.go

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: BSD-3-Clause
+
 // Code generated by controller-gen. DO NOT EDIT.
 
 package internal