Trust region strategy destructor is not virtual.

Thanks to Markus Moll for reporting this.

Change-Id: I8b7c8fa6b16a5127a2674acddb804ec5f45455ce

internal/ceres/trust_region_strategy.h

diff --git a/internal/ceres/trust_region_strategy.h b/internal/ceres/trust_region_strategy.h
index b15a895..8d5977e 100644
--- a/internal/ceres/trust_region_strategy.h
+++ b/internal/ceres/trust_region_strategy.h
@@ -79,7 +79,7 @@
     double eta;
   };
 
-  ~TrustRegionStrategy();
+  virtual ~TrustRegionStrategy();
 
   // Use the current radius to solve for the trust region step.
   virtual LinearSolver::Summary ComputeStep(