Add missing friend declaration

Eigen/src/SparseCore/SparseMatrix.h

diff --git a/Eigen/src/SparseCore/SparseMatrix.h b/Eigen/src/SparseCore/SparseMatrix.h
index 64ca5fc..ec650b2 100644
--- a/Eigen/src/SparseCore/SparseMatrix.h
+++ b/Eigen/src/SparseCore/SparseMatrix.h
@@ -94,6 +94,7 @@
 {
     typedef SparseCompressedBase<SparseMatrix> Base;
     using Base::convert_index;
+    friend class SparseVector<_Scalar,0,_Index>;
   public:
     using Base::isCompressed;
     using Base::nonZeros;