Use the correct icon name in bitcoin-qt.desktop

Summary: as per title

Test Plan: install deb package from ubuntu ppa repo and chekck if bitcoin-qt use the correct icon

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2549