Fix clang warnings regarding overrides of CCoinsView functions

Summary: Fixes clang warnings regarding overrides of CCoinsView functions.

Test Plan: make VERBOSE=1 check

Reviewers: deadalnix, freetrader, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: zquestz

Differential Revision: https://reviews.bitcoinabc.org/D548