Fix hardcoded private browsing icon

Summary:
BUG: 397078

Fixes the hardcoded private browsing icon in the Falkon application menu.

Test Plan: Open the Falkon application menu on the top right.

Reviewers: Falkon, drosca

Reviewed By: Falkon, drosca

Subscribers: ngraham, drosca, falkon, Falkon

Tags: Falkon

Differential Revision: https://phabricator.kde.org/D17288