neutrino.h: add signal OnShutDown

was missed in context with similar OnBeforeRestart


Origin commit data
------------------
Commit: 5e0137f8a6
Author: Thilo Graf <dbt@novatux.de>
Date: 2018-12-08 (Sat, 08 Dec 2018)

src/neutrino.cpp

@@ -4206,6 +4206,8 @@ void CNeutrinoApp::ExitRun(int exit_code)
 		SDT_ReloadChannels();
 
 	dprintf(DEBUG_INFO, "exit\n");
+	OnShutDown();
+
 	StopSubtitles();
 	stopPlayBack();