static cast to force Eigen::Index to long conversion.

Change-Id: I11945c455eb3fef9bfb952f5a30d657542fbccae

internal/ceres/gradient_checking_cost_function.cc

diff --git a/internal/ceres/gradient_checking_cost_function.cc b/internal/ceres/gradient_checking_cost_function.cc
index 7fb3ed7..3edf95d 100644
--- a/internal/ceres/gradient_checking_cost_function.cc
+++ b/internal/ceres/gradient_checking_cost_function.cc
@@ -154,8 +154,8 @@
                     "Jacobian for " "block %d: (%ld by %ld)) "
                     "==========\n",
                     k,
-                    term_jacobians[k].rows(),
-                    term_jacobians[k].cols());
+                    static_cast<long>(term_jacobians[k].rows()),
+                    static_cast<long>(term_jacobians[k].cols()));
       // The funny spacing creates appropriately aligned column headers.
       m += " block  row  col        user dx/dy    num diff dx/dy         "
            "abs error    relative error         parameter          residual\n";