QStateMachine: mark a ctor as explicit

Commit 0b66f723f06f6d115ea37d4db8bb6c0b5f63885b recently introduced a new
QStateMachine constructor, but failed to mark it as explicit. Fix.

Change-Id: I16037691ad77d528bb50b611c03063b17a71dd34
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

src/corelib/statemachine/qstatemachine.h

@@ -114,7 +114,7 @@ public:
     };
 
     explicit QStateMachine(QObject *parent = 0);
-    QStateMachine(QState::ChildMode childMode, QObject *parent = 0);
+    explicit QStateMachine(QState::ChildMode childMode, QObject *parent = 0);
     ~QStateMachine();
 
     void addState(QAbstractState *state);