remove a const introduced in 2008-05-02T19:58:37Z!kailoran@gmail.com, which blocked compilation

src/menu_events.cpp

@@ -1950,7 +1950,7 @@ private:
 				return get_command(cmd) != 0;
 			}
 			//actual work function
-			void dispatch(const std::string& cmd)
+			void dispatch(std::string cmd)
 			{
 				if (empty()) {
 					init_map_default();