qt: use proper include for types used in qt slot

Summary:
The forward declaration is not good enough when attempting to compile with Qt6. We get a compile error.
Depends on D18185

Test Plan:
ninja all check-all

Modify cmake config files to compile with Qt6 and see that it works (only the compilation works, the executable still doesn't run as expected)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18190