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

Eigen/src/Core/DiagonalMatrix.h

@@ -26,6 +26,8 @@
 #ifndef EIGEN_DIAGONALMATRIX_H
 #define EIGEN_DIAGONALMATRIX_H
 
+namespace Eigen { 
+
 #ifndef EIGEN_PARSED_BY_DOXYGEN
 template<typename Derived>
 class DiagonalBase : public EigenBase<Derived>
@@ -303,4 +305,6 @@ bool MatrixBase<Derived>::isDiagonal(RealScalar prec) const
   return true;
 }
 
+} // end namespace Eigen
+
 #endif // EIGEN_DIAGONALMATRIX_H