Add missing <iterator> include.

Change-Id: Ied1199569d952fcdcaa0171893631d01d40eea7f

internal/ceres/compressed_row_jacobian_writer.cc

diff --git a/internal/ceres/compressed_row_jacobian_writer.cc b/internal/ceres/compressed_row_jacobian_writer.cc
index 6ee7226..d21aa7d 100644
--- a/internal/ceres/compressed_row_jacobian_writer.cc
+++ b/internal/ceres/compressed_row_jacobian_writer.cc
@@ -30,6 +30,7 @@
 
 #include "ceres/compressed_row_jacobian_writer.h"
 
+#include <iterator>
 #include <utility>
 #include <vector>