More ClangTidy fixes

Change-Id: I8288a61cf0db80efd7fd4862a555cc717797e8ba

internal/ceres/program_test.cc

diff --git a/internal/ceres/program_test.cc b/internal/ceres/program_test.cc
index 115d905..6844168 100644
--- a/internal/ceres/program_test.cc
+++ b/internal/ceres/program_test.cc
@@ -33,6 +33,7 @@
 #include <cmath>
 #include <limits>
 #include <memory>
+#include <string>
 #include <utility>
 #include <vector>
 

internal/ceres/residual_block_utils.cc

diff --git a/internal/ceres/residual_block_utils.cc b/internal/ceres/residual_block_utils.cc
index dfb41c0..1f0eef7 100644
--- a/internal/ceres/residual_block_utils.cc
+++ b/internal/ceres/residual_block_utils.cc
@@ -33,6 +33,7 @@
 #include <cmath>
 #include <cstddef>
 #include <limits>
+#include <string>
 
 #include "ceres/array_utils.h"
 #include "ceres/internal/eigen.h"