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/stringappend.h

@@ -4,6 +4,8 @@
  * Copyright 2013 Facebook
  */
 
+#pragma once
+
 #include "leveldb/merge_operator.h"
 #include "leveldb/slice.h"
 

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"
 

utilities/redis/redis_list_iterator.h

@@ -37,6 +37,8 @@
  * Copyright 2013 Facebook
  */
 
+#pragma once
+
 #include <string>
 
 #include "redis_list_exception.h"

utilities/redis/redis_lists.h

@@ -8,6 +8,8 @@
  * Copyright 2013 Facebook
  */
 
+#pragma once
+
 #include <string>
 #include "leveldb/db.h"
 #include "redis_list_iterator.h"