Clarify pico_sdk_import.cmake license

This file was copied from the pico-sdk, which is a BSD 3-Clause project.

pico_sdk_import.cmake

diff --git a/pico_sdk_import.cmake b/pico_sdk_import.cmake
index 65f8a6f..d4773f8 100644
--- a/pico_sdk_import.cmake
+++ b/pico_sdk_import.cmake
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: BSD-3-Clause
 # This is a copy of <PICO_SDK_PATH>/external/pico_sdk_import.cmake
 
 # This can be dropped into an external project to help locate this SDK