[Qt] Fix deprecated QSignalMapper::mapped event

Summary: This is replaced by QSignalMapper::mappedType since Qt 5.15.

Test Plan:
With Qt 5.5.1 and 5.15:

ninja all check

Check the warning is gone.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6604