Add explicit license and copyright headers to external/pico_sdk_import.cmake

[P33M]

This came up in a review of debugprobe - as this file gets copied around a lot, and potentially into other projects which may have different licencing arrangements, this file needs to be specifically attributed to the original authors.

[lurch]

According to https://github.com/raspberrypi/pico-sdk/commits/develop/external/pico_sdk_import.cmake those authors are @kilograham , @willeccles and @ahmetkizilay .