add #pragma once to some header files (facebookresearch#4562)

Summary:
Pull Request resolved: facebookresearch#4562

#pragma once:
*   Prevents potential compilation errors from multiple definitions
*   Improves compilation performance by avoiding redundant processing

Reviewed By: junjieqi

Differential Revision: D81094022

fbshipit-source-id: 5a77e70e381bfcfb881cb1a132ef891fe5fc14dc

Author: limqiying

faiss/gpu/perf/IndexWrapper-inl.h

@@ -5,6 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+#pragma once
+
 #include <faiss/impl/FaissAssert.h>
 
 namespace faiss {

faiss/impl/ProductQuantizer-inl.h

@@ -5,6 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+#pragma once
+
 namespace faiss {
 
 inline PQEncoderGeneric::PQEncoderGeneric(

faiss/impl/ThreadedIndex-inl.h

@@ -5,6 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+#pragma once
+
 #include <faiss/impl/FaissAssert.h>
 #include <exception>
 #include <iostream>

faiss/utils/extra_distances-inl.h

@@ -5,6 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+#pragma once
+
 /** In this file are the implementations of extra metrics beyond L2
  *  and inner product */
 

faiss/utils/hamming-inl.h

@@ -5,6 +5,8 @@
  * LICENSE file in the root directory of this source tree.
  */
 
+#pragma once
+
 namespace faiss {
 
 // BitstringWriter and BitstringReader functions