Add missing override keyword to DummySignatureCreator::CreateSig()

Summary:
This is an ovesight from D3889 which is throwing a warning at build
time.

Test Plan:
Build and check there is no longer any -Winconsistent-missing-override
warning;

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3985