[backport#16348] gui: Fix missing qRegisterMetaType(WalletModel*)

Summary:
https://github.com/bitcoin/bitcoin/pull/16348/commits/f27bd96b5fdc2921d93c44bbf422bff0e979c4de

Depends on D6180

Partial backport of Core PR16348

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6181