QWindowsXPStyle: Enlarge QLineEdit/QComboBox when High DPI scaling is active.

In QWindowsXPStyle::sizeFromContents(), scale the margins using the
devicePixelRatio and round the height up. Typically increases
the height by 2 pixels when the device pixel ratio equals 2.

Change-Id: I446ad518c3808a6454a08db7b66a942d37c1703f
Task-number: QTBUG-49374
Reviewed-by: Alessandro Portale <alessandro.portale@theqtcompany.com>