Change MaxKeyCount type from enum to static constexpr int

The change was needed because std::fill_n(), which takes it, does not
handle enum type on VxWorks. As it was anonymous with only one member,
it could be changed to static constexpr int.

Task-number: QTBUG-115777
Change-Id: I7cb4d267ad21f6b963d851e30bc91042fbf5c15a
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/kernel/qkeysequence_p.h

@@ -35,7 +35,7 @@ struct QKeyBinding
 class QKeySequencePrivate
 {
 public:
-    enum { MaxKeyCount = 4 }; // also used in QKeySequenceEdit
+    static constexpr int MaxKeyCount = 4 ; // also used in QKeySequenceEdit
     constexpr QKeySequencePrivate() : ref(1), key{} {}
     inline QKeySequencePrivate(const QKeySequencePrivate &copy) : ref(1)
     {