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