Fix the build with Qt 5.15

Summary:
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