Add Covariance documentation to html docs.

Change-Id: I11ddc9f7069964596760c6ea4d85c44312c0a67a
diff --git a/include/ceres/ceres.h b/include/ceres/ceres.h
index b7aeeef..5dcca07 100644
--- a/include/ceres/ceres.h
+++ b/include/ceres/ceres.h
@@ -41,6 +41,7 @@
 #include "ceres/autodiff_local_parameterization.h"
 #include "ceres/cost_function.h"
 #include "ceres/cost_function_to_functor.h"
+#include "ceres/covariance.h"
 #include "ceres/crs_matrix.h"
 #include "ceres/iteration_callback.h"
 #include "ceres/jet.h"