Replace include/leveldb with include/rocksdb.

Summary: Replace include/leveldb with include/rocksdb.

Test Plan:
make clean; make check
make clean; make release

Differential Revision: https://reviews.facebook.net/D12489

utilities/merge_operators/string_append/stringappend2.h

@@ -11,9 +11,8 @@
  */
 
 #pragma once
-
-#include "leveldb/merge_operator.h"
-#include "leveldb/slice.h"
+#include "rocksdb/merge_operator.h"
+#include "rocksdb/slice.h"
 
 namespace leveldb {