Make SubsetPreconditionerTest compatible with old Eigen.

The enum Eigen::UpLoType did not have a name in older versions
of Eigen, so a templated function using that enum type fails
to compile with earlier versions of Eigen.

This change replaces the enum in the template declaration with
an int.

Change-Id: Id128fd96b76818be347ee6ed5945c231936d9af8

internal/ceres/subset_preconditioner_test.cc

diff --git a/internal/ceres/subset_preconditioner_test.cc b/internal/ceres/subset_preconditioner_test.cc
index 0f4cc6c..147ecf8 100644
--- a/internal/ceres/subset_preconditioner_test.cc
+++ b/internal/ceres/subset_preconditioner_test.cc
@@ -44,7 +44,7 @@
 
 // TODO(sameeragarwal): Refactor the following two functions out of
 // here and sparse_cholesky_test.cc into a more suitable place.
-template <Eigen::UpLoType UpLoType>
+template <int UpLoType>
 bool SolveLinearSystemUsingEigen(const Matrix& lhs,
                                  const Vector rhs,
                                  Vector* solution) {