[backport] trivial: Mark overrides as such. #13282

Summary:
60ebc7da4c trivial: Mark overrides as such. (Daniel Kraft)

Pull request description:

This trivial change adds the override keyword to some methods that override virtual base class / interface methods. This ensures that any future changes to the interface's method signatures which are not correctly mirrored in the subclasses will break at compile time with a clear error message, rather than at runtime.

This is a backport of Core PR13282

Test Plan:

ninja check
ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, nakihito, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, nakihito, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5417