simulator: flush output after waiting message output

Just to make sure the user sees the message.

src/modules/simulator/simulator_mavlink.cpp

@@ -547,6 +547,7 @@ void Simulator::pollForMAVLinkMessages(bool publish, int udp_port)
 	// this is important for the UDP communication to work
 	int pret = -1;
 	PX4_INFO("Waiting for initial data on UDP port %i. Please start the flight simulator to proceed..", udp_port);
+	fflush(stdout);
 
 	uint64_t pstart_time = 0;