Windows: Added Comma to list of possible numpad keys

Some keyboard layouts (German and Czech for example) have comma instead
of period on the numpad, so this key should also be considered when
setting the Qt::KeypadModifer state.

Task-number: QTBUG-38248
Change-Id: I06847a02a9334c21784790eae6fd7e1bc6de4099
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
(cherry picked from commit 7e44e4bb4e23758be7c05261e0413365491c6846)