message: add missing DNN header to opencv2/opencv.hpp
diff: #ifdef HAVE_OPENCV_FEATURES2D
  #include "opencv2/features2d.hpp"
  #endif
  +#ifdef HAVE_OPENCV_DNN
  +#include "opencv2/dnn.hpp"
  +#endif
  #ifdef HAVE_OPENCV_FLANN
  #include "opencv2/flann.hpp"
  #endif
  