Add SPDX tags to get_cpm.cmake to comply with MIT (#496)

Add [SPDX][1] licensing/copyright tags to the `get_cpm.cmake` script,
so that anybody that copies the `get_cpm.cmake` script into their repo
(following the instructions in the README.md)
will automatically comply with CPM's MIT license.

[1]: https://spdx.dev/

cmake/get_cpm.cmake

@@ -1,3 +1,7 @@
+# SPDX-License-Identifier: MIT
+#
+# SPDX-FileCopyrightText: Copyright (c) 2019-2022 Lars Melchior and contributors
+
 set(CPM_DOWNLOAD_VERSION 1.0.0-development-version)
 
 if(CPM_SOURCE_CACHE)