Mark get_clock() as override to fix clang warnings. (#939)

Signed-off-by: Steven! Ragnarök <steven@nuclearsandwich.com>

rclcpp/include/rclcpp/node_interfaces/node_clock.hpp

@@ -56,9 +56,8 @@ public:
 
   /// Get a clock which will be kept up to date by the node.
   RCLCPP_PUBLIC
-  virtual
   rclcpp::Clock::ConstSharedPtr
-  get_clock() const;
+  get_clock() const override;
 
 private:
   RCLCPP_DISABLE_COPY(NodeClock)