Fix a fuzzer build failure and run the build on CI

Summary:
The issue was introduced silently in a recent backport and causes the
fuzzer to fail the build.
This diff fixes the issue and run the build on CI to prevent future
breakage.

Test Plan:

ninja build-fuzzers

./contrib/teamcity/build-configurations.py build-fuzzer

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8634