Add destructor to Covariance.

This allows CovarianceImpl to be forward declared without
scoped_ptr freaking out.

Thanks to Nima Keivan for reporting this.

Change-Id: Icd5aa766b3aab70246055225231a4b971c6b7b90
diff --git a/include/ceres/covariance.h b/include/ceres/covariance.h
index 1e9bb9f..6533487 100644
--- a/include/ceres/covariance.h
+++ b/include/ceres/covariance.h
@@ -234,6 +234,7 @@
   };
 
   explicit Covariance(const Options& options);
+  ~Covariance();
 
   // Compute a part of the covariance matrix.
   //