MB-59575: Refactor member variables alignment of IndexFlatCodes (#22)

- moving the codes_ptr down below so that we skip compiler optimization
  for that pointer member variable

Author: Thejas-bhat

faiss/IndexFlatCodes.h

@@ -24,9 +24,9 @@ struct IndexFlatCodes : Index {
 
     /// encoded dataset, size ntotal * code_size
     std::vector<uint8_t> codes;
-    uint8_t* codes_ptr;
     bool mmaped; // true if codes_ptr is pointing to a mmaped region
     size_t mmaped_size;
+    uint8_t* codes_ptr;
 
     IndexFlatCodes();