ostree_repo_static_delta_generate: Fix leak

There is no need to ref the argument of g_variant_builder_add_value

Closes: #1701
Approved by: jlebon

src/libostree/ostree-repo-static-delta-compilation.c

@@ -1521,7 +1521,7 @@ ostree_repo_static_delta_generate (OstreeRepo                   *self,
             goto out;
         }
 
-      g_variant_builder_add_value (part_headers, g_variant_ref (part_builder->header));
+      g_variant_builder_add_value (part_headers, part_builder->header);
 
       total_compressed_size += part_builder->compressed_size;
       total_uncompressed_size += part_builder->uncompressed_size;