Fix memory leak in stdout journal streams

Just as 'identifier' is strdup-ed and freed, we need to do the same for
unit_id.

src/journal/journald-stream.c

@@ -340,6 +340,7 @@ void stdout_stream_free(StdoutStream *s) {
 #endif
 
         free(s->identifier);
+        free(s->unit_id);
         free(s);
 }