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

Eigen/src/Core/Diagonal.h

@@ -26,6 +26,8 @@
 #ifndef EIGEN_DIAGONAL_H
 #define EIGEN_DIAGONAL_H
 
+namespace Eigen { 
+
 /** \class Diagonal
   * \ingroup Core_Module
   *
@@ -245,4 +247,6 @@ MatrixBase<Derived>::diagonal() const
   return derived();
 }
 
+} // end namespace Eigen
+
 #endif // EIGEN_DIAGONAL_H