Restore missing operator=

Author: gnawme

apps/point_cloud_editor/include/pcl/apps/point_cloud_editor/statistics.h

@@ -67,7 +67,7 @@ class Statistics {
 
   /// @brief Equal Operator
   Statistics&
-  operator=(const Statistics&);
+  operator=(const Statistics&) = default;
 
   /// @brief Returns the statistics in string.
   virtual std::string