Fixed spare unit test.

test/sparse.h

diff --git a/test/sparse.h b/test/sparse.h
index 4733e4d..246dc1f 100644
--- a/test/sparse.h
+++ b/test/sparse.h
@@ -41,6 +41,7 @@
 #include <Eigen/Cholesky>
 #include <Eigen/LU>
 #include <Eigen/Sparse>
+#include <Eigen/SparseExtra>
 
 enum {
   ForceNonZeroDiag = 1,