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

include/ceres/covariance.h

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.
   //

internal/ceres/covariance.cc

diff --git a/internal/ceres/covariance.cc b/internal/ceres/covariance.cc
index 6cb52ee..35146c5 100644
--- a/internal/ceres/covariance.cc
+++ b/internal/ceres/covariance.cc
@@ -42,6 +42,9 @@
   impl_.reset(new internal::CovarianceImpl(options));
 }
 
+Covariance::~Covariance() {
+}
+
 bool Covariance::Compute(
     const vector<pair<const double*, const double*> >& covariance_blocks,
     Problem* problem) {