Fix a clang warning

Change-Id: I5ef32c6329f1f75efb30b16519b8de146a8339fa

internal/ceres/solver.cc

diff --git a/internal/ceres/solver.cc b/internal/ceres/solver.cc
index 45e6865..6561bd9 100644
--- a/internal/ceres/solver.cc
+++ b/internal/ceres/solver.cc
@@ -336,8 +336,8 @@
                     initial_cost - final_cost);
     }
 
-    StringAppendF(&report, "\nNumber of iterations:    % 20ld\n",
-                  iterations.size() - 1);
+    StringAppendF(&report, "\nNumber of iterations:    % 20d\n",
+                  static_cast<int>(iterations.size() - 1));
 
     StringAppendF(&report, "\nTime (in seconds):\n");
     StringAppendF(&report, "Preprocessor        %25.3f\n",