Support opencv 5

[linsein]

I build frei0r 3.2.3 on Alpine Linux with opencv 5.0.0 and get a compile error:

frei0r-3.2.3/src/filter/facebl0r/facebl0r.cpp:67:9: error: 'CascadeClassifier' in namespace 'cv' does not name a type
   67 |     cv::CascadeClassifier cascade;
      |         ^~~~~~~~~~~~~~~~~

According https://github.com/opencv/opencv/wiki/OpenCV-4-to-5-migration#objdetect-haar-and-hog-moved-to-contrib,
CascadeClassifier is moved to the xobjdetect module in opencv_contrib, which header file is no included in <opencv2/opencv.h>.