Add missing ceres/internal/port.h to two header files.

Without these these files cannot be included on their own.

https://github.com/ceres-solver/ceres-solver/issues/410

Change-Id: I87ca91d15f5ab2053e43480ecbde74779125c709

include/ceres/evaluation_callback.h

diff --git a/include/ceres/evaluation_callback.h b/include/ceres/evaluation_callback.h
index 46a42af..a0a8601 100644
--- a/include/ceres/evaluation_callback.h
+++ b/include/ceres/evaluation_callback.h
@@ -31,6 +31,8 @@
 #ifndef CERES_PUBLIC_EVALUATION_CALLBACK_H_
 #define CERES_PUBLIC_EVALUATION_CALLBACK_H_
 
+#include "ceres/internal/port.h"
+
 namespace ceres {
 
 // Using this callback interface, Ceres can notify you when it is about to

include/ceres/numeric_diff_options.h

diff --git a/include/ceres/numeric_diff_options.h b/include/ceres/numeric_diff_options.h
index 3356be9..a77be44 100644
--- a/include/ceres/numeric_diff_options.h
+++ b/include/ceres/numeric_diff_options.h
@@ -32,6 +32,8 @@
 #ifndef CERES_PUBLIC_NUMERIC_DIFF_OPTIONS_H_
 #define CERES_PUBLIC_NUMERIC_DIFF_OPTIONS_H_
 
+#include "ceres/internal/port.h"
+
 namespace ceres {
 
 // Options pertaining to numeric differentiation (e.g., convergence criteria,