Fix integer conversion warning in MSVC.

- Raised by cnovel as issue #346:
  https://github.com/ceres-solver/ceres-solver/issues/346.

Change-Id: I90815ab6412834dd793ca7c327002d551da0675b

include/ceres/dynamic_autodiff_cost_function.h

diff --git a/include/ceres/dynamic_autodiff_cost_function.h b/include/ceres/dynamic_autodiff_cost_function.h
index 1cece2c..4a31236 100644
--- a/include/ceres/dynamic_autodiff_cost_function.h
+++ b/include/ceres/dynamic_autodiff_cost_function.h
@@ -105,7 +105,8 @@
     // To work around this issue, the solution here is to evaluate the
     // jacobians in a series of passes, each one computing Stripe *
     // num_residuals() derivatives. This is done with small, fixed-size jets.
-    const int num_parameter_blocks = parameter_block_sizes().size();
+    const int num_parameter_blocks =
+        static_cast<int>(parameter_block_sizes().size());
     const int num_parameters = std::accumulate(parameter_block_sizes().begin(),
                                                parameter_block_sizes().end(),
                                                0);