QmlDesigner: Fix compile for Qt5.11

Amends 4176b17fff.

Change-Id: Ib9a95757bc4cbcf8ce13c432e5fda8b95282936b
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/plugins/qmldesigner/components/propertyeditor/gradientpresetitem.h

@@ -42,7 +42,7 @@ public:
 #if QT_VERSION >= QT_VERSION_CHECK(5, 12, 0)
     using Preset = QGradient::Preset;
 #else
-    enum Preset {};
+    enum Preset { NumPresets = 0 };
 #endif
 
     explicit GradientPresetItem();