Make SdlThread constructor explicit

Source/dthread.cpp

@@ -86,7 +86,7 @@ void dthread_start()
 	DthreadRunning = true;
 	DthreadMutex.emplace();
 	WorkToDo.emplace();
-	Thread = { DthreadHandler };
+	Thread = SdlThread { DthreadHandler };
 }
 
 void DThreadCleanup()

Source/nthread.cpp

@@ -181,7 +181,7 @@ void nthread_start(bool setTurnUpperBit)
 		sgbThreadIsRunning = false;
 		MemCrit.lock();
 		nthread_should_run = true;
-		Thread = { NthreadHandler };
+		Thread = SdlThread { NthreadHandler };
 	}
 }
 

Source/utils/sdl_thread.h

@@ -34,7 +34,7 @@ public:
 			ErrSdl();
 	}
 
-	SdlThread(void (*handler)(void))
+	explicit SdlThread(void (*handler)(void))
 	    : SdlThread(ThreadTranslate, (void *)handler)
 	{
 	}