Add missing override keyword to DummySignatureCreator::CreateSig()
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

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