fix the cpplint errors

Author: Plumezz

example_external/include/generate_parameter_library_example_external/minimal_publisher_external.hpp

@@ -28,6 +28,7 @@
 
 #pragma once
 
+#include <memory>
 #include <rclcpp/rclcpp.hpp>
 #include <rclcpp_components/register_node_macro.hpp>
 
@@ -37,7 +38,7 @@ namespace admittance_controller {
 
 class MinimalPublisher : public rclcpp::Node {
  public:
-  MinimalPublisher(const rclcpp::NodeOptions& options = rclcpp::NodeOptions());
+  explicit MinimalPublisher(const rclcpp::NodeOptions& options = rclcpp::NodeOptions());
 
  private:
   void timer_callback();