[LINTER] Fix syntax error in the cppcheck linter

Summary:
Interestingly this extra comma is not failing on most platforms with
recent php, but it fails on Ubuntu Bionic (with PHP 7.2) at least.

Test Plan:
On Ubuntu Bionic:

arc lint

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6992