do not monitor mpdclient file descriptor with select() if idle is off

it doesn't do anything in such case.

src/ncmpcpp.cpp

@@ -275,6 +275,7 @@ int main(int argc, char *argv[])
 	wFooter = new Window(0, footer_start_y, COLS, footer_height, "", Config.statusbar_color, brNone);
 	wFooter->SetTimeout(ncmpcpp_window_timeout);
 	wFooter->SetGetStringHelper(StatusbarGetStringHelper);
+	if (Mpd.SupportsIdle())
 		wFooter->AddFDCallback(Mpd.GetFD(), StatusbarMPDCallback);
 	wFooter->CreateHistory();
 	
@@ -333,6 +334,7 @@ int main(int argc, char *argv[])
 			if (Mpd.Connect())
 			{
 				ShowMessage("Connected to %s!", Mpd.GetHostname().c_str());
+				if (Mpd.SupportsIdle())
 					wFooter->AddFDCallback(Mpd.GetFD(), StatusbarMPDCallback);
 				MessagesAllowed = 0;
 				UpdateStatusImmediately = 1;