Fix compilation for MSVC.

unsupported/Eigen/src/NumericalDiff/NumericalDiff.h

diff --git a/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h b/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
index 98872e0..db6f791 100644
--- a/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
+++ b/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
@@ -45,10 +45,11 @@
   *
   * Currently only "Forward" and "Central" scheme are implemented.
   */
-template<typename Functor, NumericalDiffMode mode=Forward>
-class NumericalDiff : public Functor
+template<typename _Functor, NumericalDiffMode mode=Forward>
+class NumericalDiff : public _Functor
 {
 public:
+    typedef _Functor Functor;
     typedef typename Functor::Scalar Scalar;
     typedef typename Functor::InputType InputType;
     typedef typename Functor::ValueType ValueType;