Add missing Q_ENUM to QAbstractSpinBox::StepType

Fix designer startup warning:
QMetaProperty::read: Unable to handle unregistered datatype 'StepType' for property 'QSpinBox::stepType'
QMetaProperty::read: Unable to handle unregistered datatype 'StepType' for property 'QDoubleSpinBox::stepType'

Amends 97e40a5409.

Change-Id: If99d118fcda0f8420bb0c3424427ac4f2c48505e
Reviewed-by: Harald Nordgren <haraldnordgren@gmail.com>
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/widgets/widgets/qabstractspinbox.h

@@ -132,6 +132,8 @@ public:
         DefaultStepType,
         AdaptiveDecimalStepType
     };
+    Q_ENUM(StepType)
+
 public Q_SLOTS:
     void stepUp();
     void stepDown();