Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/scheduler.h

@@ -102,7 +102,7 @@ private:
     void ProcessQueue();
 
 public:
-    SingleThreadedSchedulerClient(CScheduler *pschedulerIn) : m_pscheduler(pschedulerIn) {}
+    explicit SingleThreadedSchedulerClient(CScheduler *pschedulerIn) : m_pscheduler(pschedulerIn) {}
     void AddToProcessQueue(std::function<void (void)> func);
 
     // Processes all remaining queue members on the calling thread, blocking until queue is empty