CMake: add spdx tags to modules

Helps to be explict what license each module is under

Signed-off-by: Thomas Ingleby <thomas.ingleby@intel.com>

cmake/modules/GetGitRevisionDescription.cmake.in

@@ -12,6 +12,8 @@
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or copy at
 # http://www.boost.org/LICENSE_1_0.txt)
+#
+# SPDX-License-Identifier: BSL-1.0
 
 set(HEAD_HASH)