Fix use uninitialized, detected by GCC 4.8

If the widget \a field is not present on this form, getWidgetPosition
might not fill in the role variable. GCC is correct.

qformlayout.cpp:1690:19: error: ‘role’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

Change-Id: Ia67991a71e8f1ceacb9d6370c7028c454ef630c6
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/widgets/kernel/qformlayout.cpp

@@ -1683,7 +1683,7 @@ QWidget *QFormLayout::labelForField(QWidget *field) const
     Q_D(const QFormLayout);
 
     int row;
-    ItemRole role;
+    ItemRole role = LabelRole;
 
     getWidgetPosition(field, &row, &role);