Fix operator= ambiguity on some versions of Clang.

- Earlier versions of Clang (up to at least v3.0) throw an ambiguous
  operator= error in this assignment.  Variations on this error have
  cropped up occassionally in some other projects (e.g. PCL).

Change-Id: I73a632c43528eb69840ce697c55d9afc5f3d8e59

include/ceres/dynamic_numeric_diff_cost_function.h

diff --git a/include/ceres/dynamic_numeric_diff_cost_function.h b/include/ceres/dynamic_numeric_diff_cost_function.h
index 9f70000..7216c8e 100644
--- a/include/ceres/dynamic_numeric_diff_cost_function.h
+++ b/include/ceres/dynamic_numeric_diff_cost_function.h
@@ -203,7 +203,7 @@
       // 1. Store residuals for the forward part.
       // 2. Subtract residuals for the backward (or 0) part.
       // 3. Divide out the run.
-      parameter_jacobian.col(j) = residuals;
+      parameter_jacobian.col(j).matrix() = residuals;
 
       double one_over_h = 1 / step_size(j);
       if (method == CENTRAL) {