message: Added export functions for ObjC classes
diff: #import <ImageIO/ImageIO.h>
  #include "opencv2/core.hpp"
  +#define OPENCV_OBJC_EXPORT __attribute__((visibility("default")))
  +
  //! @addtogroup videoio_ios
  //! @{
  @class CvAbstractCamera;
  -@interface CvAbstractCamera : NSObject
  +OPENCV_OBJC_EXPORT @interface CvAbstractCamera : NSObject
  {
  UIDeviceOrientation currentDeviceOrientation;
  @class CvVideoCamera;
  -@protocol CvVideoCameraDelegate <NSObject>
  +OPENCV_OBJC_EXPORT @protocol CvVideoCameraDelegate <NSObject>
  #ifdef __cplusplus
  // delegate method for processing image frames
  @end
  -@interface CvVideoCamera : CvAbstractCamera<AVCaptureVideoDataOutputSampleBufferDelegate>
  +OPENCV_OBJC_EXPORT @interface CvVideoCamera : CvAbstractCamera<AVCaptureVideoDataOutputSampleBufferDelegate>
  {
  AVCaptureVideoDataOutput *videoDataOutput;
  @class CvPhotoCamera;
  -@protocol CvPhotoCameraDelegate <NSObject>
  +OPENCV_OBJC_EXPORT @protocol CvPhotoCameraDelegate <NSObject>
  - (void)photoCamera:(CvPhotoCamera*)photoCamera capturedImage:(UIImage *)image;
  - (void)photoCameraCancel:(CvPhotoCamera*)photoCamera;
  @end
  -@interface CvPhotoCamera : CvAbstractCamera
  +OPENCV_OBJC_EXPORT @interface CvPhotoCamera : CvAbstractCamera
  {
  AVCaptureStillImageOutput *stillImageOutput;
  }
  