Fix missing newline at end of keys file
Summary:
check-keys fails to process the last line if it does not
end in a newline.
Test Plan: check-keys.sh
Reviewers: #bitcoin_abc, deadalnix, nakihito, Fabien
Reviewed By: #bitcoin_abc, nakihito, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D4805