main: fix debug message

If plymouthd is already running, then a second invocation will
fail with the message:

could not log bootup

This message is misleading, so fix it.

src/main.c

@@ -2496,7 +2496,7 @@ main (int    argc,
 
   if (state.boot_server == NULL)
     {
-      ply_trace ("could not log bootup: %m");
+      ply_trace ("plymouthd is already running");
 
       detach_from_running_session (&state);