Disabled warning C4251 Added the files disable_warnings.h and reenable_warnings.h which need to be included by every file that uses the macro CERES_EXPORT.

Change-Id: I176326a600d094a4524bac873564fcbf8efd2456
diff --git a/include/ceres/cost_function.h b/include/ceres/cost_function.h
index fee3e73..45292ec 100644
--- a/include/ceres/cost_function.h
+++ b/include/ceres/cost_function.h
@@ -48,6 +48,7 @@
 #include "ceres/internal/macros.h"
 #include "ceres/internal/port.h"
 #include "ceres/types.h"
+#include "ceres/internal/disable_warnings.h"
 
 namespace ceres {
 
@@ -142,4 +143,6 @@
 
 }  // namespace ceres
 
+#include "ceres/internal/reenable_warnings.h"
+
 #endif  // CERES_PUBLIC_COST_FUNCTION_H_