ClangTidy fixes

Change-Id: I43badfd6706a44148989ac5621134453bad28133

internal/ceres/block_sparse_matrix.cc

diff --git a/internal/ceres/block_sparse_matrix.cc b/internal/ceres/block_sparse_matrix.cc
index df2d9e1..6feedc8 100644
--- a/internal/ceres/block_sparse_matrix.cc
+++ b/internal/ceres/block_sparse_matrix.cc
@@ -33,6 +33,7 @@
 #include <algorithm>
 #include <cstddef>
 #include <memory>
+#include <numeric>
 #include <random>
 #include <vector>