qt: Fix Window -> Minimize menu item on linux

Summary:
On linux, the "Minimize" menu icon worked with using the Ctrl + M shortcut, but not with a click on the action in the menu. This fixes it.

Backport of PR18549

Depends on D7894

Test Plan:
ninja && ninja check

Run src/qt/bitcoin-qt and check that the Window -> Minimize menu action works on linux, both via the keyboard shortcut and with a click in the menu.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7895