<iterator> needed for back_insert_iterator

Adding this header was required to make Ceres compile with VS2010.

Change-Id: I000c860da4fd385d625e70695564225bdfd433c7

examples/nist.cc

diff --git a/examples/nist.cc b/examples/nist.cc
index ec8c56d..8e0f37e 100644
--- a/examples/nist.cc
+++ b/examples/nist.cc
@@ -72,6 +72,7 @@
 //      Winner       0        0         5   11        2     41
 
 #include <iostream>
+#include <iterator>
 #include <fstream>
 #include "ceres/ceres.h"
 #include "gflags/gflags.h"