Fix the build

Summary:
It was broken by rABCd5f1b85c7f9aace2c69e88f8a4bd655a01964b5e
See T25 for details.

Test Plan:

make

Reviewers: freetrader, kyuupichan, sickpig, awemany, #bitcoin_abc

Reviewed By: kyuupichan, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D175