Added include guards to stringappend and redis-list

Summary: added "#pragma once" in the .h files

Test Plan: make and run: stringappend_test, redis_test

Reviewers: emayanke, haobo

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12495

utilities/merge_operators/string_append/stringappend2.h

@@ -10,6 +10,8 @@
  * Copyright 2013 Facebook
  */
 
+#pragma once
+
 #include "leveldb/merge_operator.h"
 #include "leveldb/slice.h"