This also fixes the auto test, tst_QKeySequence::parseString() with
Win+A and Simon+G.
[ChangeLog][QtGui][OS X][QKeySequence] return Qt::Key_unknown with
invalid modifiers on OS X
Task-number: QTBUG-24406
Change-Id: Ie90393c9691f443c7c359cb3a487609a9691bc44
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>