Vertical align right-aligned header text

Since e907306b411 ("Right align header text that also has right-aligned
row text"), right-aligned elements are no longer centered vertically.
This can be noticed with some Qt styles (e.g. Breeze).
<pre>qtlibtorrent/torrentmodel.cpp: In member function ‘virtual QVariant TorrentModel::headerData(int, Qt::Orientation, int) const’:
qtlibtorrent/torrentmodel.cpp:405:32: error: could not convert ‘operator|(QFlags<Qt::AlignmentFlag>::enum_type, QFlags<Qt::AlignmentFlag>::enum_type)((Qt::AlignmentFlag)128u)’ from ‘QFlags<Qt::AlignmentFlag>’ to ‘QVariant’
         return (Qt::AlignRight | Qt::AlignVCenter);
                                ^</pre>