[backport] Fix the build with Qt 5.15
A fresh update to Qt 5.15 caused my build to fail. There are also some new deprecation that need to be fixed in another diff.
Test Plan: With Qt 5.15: ninja all check Check Qt 5.5.1: ninja all check
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D6362
Closes #126 (closed)