Move qNfcChecksum() definition into a .cpp file

The function is public; doesn't make sense to implement it in a private
header.

This patch also resolves the multiply-defined-symbols issue when trying
to build some Qt NFC autotests without the "QtNfc" namespace.

Change-Id: I4ba1dc31d93bb66454c7dcd043c18f81dc2ad680
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>