trivial: Improve include comment in src/interfaces/wallet.h
Summary:
Backport of Bitcoin Core PR13026
https://github.com/bitcoin/bitcoin/pull/13026
Test Plan:
make check
Reviewers: Fabien, O1 Bitcoin ABC, #bitcoin_abc, deadalnix
Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D3930