clang-tidy: use = default

Found with modernize-use-equals-default

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/commands/Command.h

@@ -53,7 +53,7 @@ public:
   };
 
   Command ();
-  virtual ~Command ();
+  virtual ~Command () = default;
 
   static void factory (std::map <std::string, Command*>&);