fix race condition with timestamp

plugins/inputs/rss/rss.go

@@ -47,10 +47,13 @@ func (r *RSS) Gather(acc telegraf.Accumulator) error {
 	}
 
 	// remember last time started
-	ts = time.Now()
+	tsl := time.Now()
 
 	wg.Wait()
 
+	// set last time started
+	ts = tsl
+
 	return nil
 }