Fix height of combobox in macstyle without a widget

When no widget is provided we hardcode the height and the value
was off by 2. There is no change when a widget is provided.

Change-Id: I555b5206b8750db06595a1e2572a3f8212635a9d
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>

src/widgets/styles/qmacstyle_mac.mm

@@ -6252,6 +6252,7 @@ QSize QMacStyle::sizeFromContents(ContentsType ct, const QStyleOption *opt,
         return sz;
     case CT_ComboBox:
         sz.rwidth() += 50;
+        sz.rheight() += 2;
         break;
     case CT_Menu: {
         QStyleHintReturnMask menuMask;