Add a default value for Solver::Summary::linear_solver_ordering_type

Change-Id: I5c7c9acdc37ba0755d479ac9c6c2872fe89fb492

include/ceres/solver.h

diff --git a/include/ceres/solver.h b/include/ceres/solver.h
index 43b2e1a..68438a1 100644
--- a/include/ceres/solver.h
+++ b/include/ceres/solver.h
@@ -1004,7 +1004,7 @@
 
     bool mixed_precision_solves_used = false;
 
-    LinearSolverOrderingType linear_solver_ordering_type;
+    LinearSolverOrderingType linear_solver_ordering_type = AMD;
 
     // Size of the elimination groups given by the user as hints to
     // the linear solver.