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

src/lib/tools/iconprovider.cpp

@@ -154,7 +154,7 @@ QIcon IconProvider::newWindowIcon()
 
 QIcon IconProvider::privateBrowsingIcon()
 {
-    return QIcon(QSL(":/icons/menu/privatebrowsing.png"));
+    return QIcon::fromTheme(QSL("view-private-symbolic"), QIcon(QSL(":/icons/menu/privatebrowsing.png")));
 }
 
 QIcon IconProvider::settingsIcon()