Fix access specifier in PointCloudColorHandlerRGBAField

[UnaNancyOwen]

Fix access specifier in PointCloudColorHandlerRGBAField.
I think that members derived from the base class should be private in this class.
(See PointCloudColorHandlerRGBField a similar class.)

• PointCloudColorHandlerRGBField
  

  pcl/visualization/include/pcl/visualization/point_cloud_color_handlers.h

  Lines 301 to 312 in 377a5bd

  	protected:
  	/** \brief Class getName method. */
  	virtual std::string
  	getName () const { return ("PointCloudColorHandlerRGBField"); }
  	private:
  	// Members derived from the base class
  	using PointCloudColorHandler<PointT>::cloud_;
  	using PointCloudColorHandler<PointT>::capable_;
  	using PointCloudColorHandler<PointT>::field_idx_;
  	using PointCloudColorHandler<PointT>::fields_;
  	};

• PointCloudColorHandlerRGBAField
  

  pcl/visualization/include/pcl/visualization/point_cloud_color_handlers.h

  Lines 486 to 496 in 377a5bd

  	protected:
  	/** \brief Class getName method. */
  	virtual std::string
  	getName () const { return ("PointCloudColorHandlerRGBAField"); }
  	// Members derived from the base class
  	using PointCloudColorHandler<PointT>::cloud_;
  	using PointCloudColorHandler<PointT>::capable_;
  	using PointCloudColorHandler<PointT>::field_idx_;
  	using PointCloudColorHandler<PointT>::fields_;
  	};