Fix narrowing conversion in LineAnnotationWidget::endStyleIcon

Summary:
Windows MSVC Qt5.11 build fails:

conversion from 'initializer list' to 'QSizeF' requires a narrowing conversion

Pass QSize, and let QSizeF Ctor QSizeF(const QSize &) do the conversion.

Test Plan: - build on Windows MSVC

Reviewers: aacid

Reviewed By: aacid

Subscribers: aacid, okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D21543