Compilation fix for msvc2010

Usage of back_inserter requires <iterator> header when using msvc2010

Change-Id: I92ee1649795ce0468ce337fc414eb0ca6e90c51e

internal/ceres/problem_impl.cc

diff --git a/internal/ceres/problem_impl.cc b/internal/ceres/problem_impl.cc
index 519e12b..2972443 100644
--- a/internal/ceres/problem_impl.cc
+++ b/internal/ceres/problem_impl.cc
@@ -33,6 +33,7 @@
 
 #include <algorithm>
 #include <cstddef>
+#include <iterator>
 #include <set>
 #include <string>
 #include <utility>