[Qt] Fix deprecated pixmap() return by pointer with Qt 5.15

Summary:
The pixmap() method returning a QImage * is deprecated with Qt 5.15.
It is replaced by a new pixmap(Qt::ReturnByValue) call which returns
by value as its name implies.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6573