create-sets.sh: Add missing copyright header

I wrote this file and am the only person who has modified it since
it was added.

MFC after:	3 days

Author: llfw

release/packages/create-sets.sh

@@ -1,4 +1,20 @@
 #! /bin/sh
+#
+# SPDX-License-Identifier: ISC
+#
+# Copyright (c) 2025 Lexi Winter <[email protected]>
+#
+# Permission to use, copy, modify, and distribute this software for any
+# purpose with or without fee is hereby granted, provided that the above
+# copyright notice and this permission notice appear in all copies.
+#
+# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+# OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 
 # Generate metapackage sets.  We do this by examining the annotations field
 # of the packages we previously built.