Get rid of include directives inside namespace blocks (bug #339).

unsupported/Eigen/src/NumericalDiff/NumericalDiff.h

diff --git a/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h b/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
index 52dc0ec..36df9f5 100644
--- a/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
+++ b/unsupported/Eigen/src/NumericalDiff/NumericalDiff.h
@@ -28,6 +28,8 @@
 #ifndef EIGEN_NUMERICAL_DIFF_H
 #define EIGEN_NUMERICAL_DIFF_H
 
+namespace Eigen { 
+
 enum NumericalDiffMode {
     Forward,
     Central
@@ -134,6 +136,8 @@
     NumericalDiff& operator=(const NumericalDiff&);
 };
 
+} // end namespace Eigen
+
 //vim: ai ts=4 sts=4 et sw=4
 #endif // EIGEN_NUMERICAL_DIFF_H