Add missing release flag to the debian package build

Summary: This is causing the software to output warnings, which it should not.

Test Plan:
Built on my private repo
https://launchpad.net/~fabcien/+archive/ubuntu/bitcoin-abc/+build/19947305,
instsalled and checked for the absence of warning.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7477