Fix header content size when sorting is disabled

Summary: This fixes the same problem as described in D4406 for Breeze.

Test Plan:
    #include <QApplication>
    #include <QTreeView>
    #include <QStandardItemModel>
    #include <QStandardItem>

    int main(int argc, char *argv[])
    {
        QApplication app(argc, argv);

        // Test model
        QStandardItemModel model(3, 2);
        model.setHorizontalHeaderLabels({ "Header 1", "Header 2" });
        for (int row = 0; row < 3; ++row) {
            for (int column = 0; column < 2; ++column) {
                model.setItem(row, column, new QStandardItem("Foo"));
            }
        }

        // View with sorting disabled
        QTreeView view;
        view.setWindowTitle("Sorting Disabled");
        view.setModel(&model);
        view.show();
        view.resizeColumnToContents(0);

        // View with sorting enabled
        QTreeView viewWithSorting;
        viewWithSorting.setWindowTitle("Sorting Enabled");
        viewWithSorting.setModel(&model);
        viewWithSorting.setSortingEnabled(true);
        viewWithSorting.show();
        viewWithSorting.resizeColumnToContents(0);

        return app.exec();
    }

Before the fix:

{F2406388}

After the fix:

{F2406390}

Reviewers: #breeze, cfeck

Reviewed By: cfeck

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4410