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

@@ -234,6 +234,7 @@ class Covariance {
   };
 
   explicit Covariance(const Options& options);
+  ~Covariance();
 
   // Compute a part of the covariance matrix.
   //

internal/ceres/covariance.cc

@@ -42,6 +42,9 @@ Covariance::Covariance(const Covariance::Options& options) {
   impl_.reset(new internal::CovarianceImpl(options));
 }
 
+Covariance::~Covariance() {
+}
+
 bool Covariance::Compute(
     const vector<pair<const double*, const double*> >& covariance_blocks,
     Problem* problem) {