Add missing includes

pair_hash.h uses std::size_t and std::hash but does not include the
corresponding headers <cstddef> and <functional>.

Change-Id: I194a5c76e8f50b1574e1359f616351581033c576

internal/ceres/pair_hash.h

diff --git a/internal/ceres/pair_hash.h b/internal/ceres/pair_hash.h
index 1e7ddeb..83ff2b4 100644
--- a/internal/ceres/pair_hash.h
+++ b/internal/ceres/pair_hash.h
@@ -1,5 +1,5 @@
 // Ceres Solver - A fast non-linear least squares minimizer
-// Copyright 2018 Google Inc. All rights reserved.
+// Copyright 2022 Google Inc. All rights reserved.
 // http://ceres-solver.org/
 //
 // Redistribution and use in source and binary forms, with or without
@@ -33,7 +33,9 @@
 #ifndef CERES_INTERNAL_PAIR_HASH_H_
 #define CERES_INTERNAL_PAIR_HASH_H_
 
+#include <cstddef>
 #include <cstdint>
+#include <functional>
 #include <utility>
 
 #include "ceres/internal/export.h"