Fix OS X style MDI area subwindow resize

Fixed regression bug of the commit #fc11798

Change-Id: I3d64fd67dfe2196a726886a19b9510dd12ff255d
Task-number: QTBUG-43392
Reviewed-by: Andy Shaw <andy.shaw@digia.com>

src/widgets/styles/qmacstyle_mac.mm

@@ -879,6 +879,10 @@ static QSize qt_aqua_get_known_size(QStyle::ContentsType ct, const QWidget *widg
             gbi.size = sz == QAquaSizeSmall ? kHIThemeGrowBoxSizeSmall : kHIThemeGrowBoxSizeNormal;
             if (HIThemeGetGrowBoxBounds(&p, &gbi, &r) == noErr) {
                 int width = 0;
+#ifndef QT_NO_MDIAREA
+            if (widg && qobject_cast<QMdiSubWindow *>(widg->parentWidget()))
+                width = r.size.width;
+#endif
                 ret = QSize(width, r.size.height);
             }
         }