mplayer: print a message when writing watch_later config for resume

Putting this into stable, because it might help debugging certain
issues, and also making the user aware what's happening.

mpvcore/mplayer.c

@@ -867,6 +867,8 @@ void mp_write_watch_later_conf(struct MPContext *mpctx)
     if (!conffile)
         goto exit;
 
+    mp_msg(MSGT_CPLAYER, MSGL_INFO, "Saving state.\n");
+
     FILE *file = fopen(conffile, "wb");
     if (!file)
         goto exit;