Use opencv2/xobjdetect.hpp for CascadeClassifier under OpenCV 5.

OpenCV 5 split the classic Haar/LBP CascadeClassifier and HOGDescriptor
out of the core 'objdetect' module into the contrib 'xobjdetect' module.
objdetect.hpp (pulled in transitively by opencv2/opencv.hpp) no longer
declares cv::CascadeClassifier, so facedetect/facebl0r fail to build.
Pull in the xobjdetect header explicitly, guarded by the major version so
OpenCV 2/3/4 (where the header does not exist) keep building unchanged.

Not yet fixed upstream as of v3.1.3 (last OpenCV work was for OpenCV 4).

--- a/src/filter/facedetect/facedetect.cpp	2026-06-06 23:50:07.618842383 -0300
+++ b/src/filter/facedetect/facedetect.cpp	2026-06-06 23:50:07.619590586 -0300
@@ -21,6 +21,12 @@
 #include <stdio.h>
 #include <string.h>
 #include <opencv2/opencv.hpp>
+#if CV_VERSION_MAJOR >= 5
+// OpenCV 5 moved CascadeClassifier/HOGDescriptor from objdetect to the
+// contrib xobjdetect module; objdetect.hpp (pulled by opencv.hpp) no longer
+// declares them. https://github.com/opencv/opencv/ (4 -> 5 migration)
+#include <opencv2/xobjdetect.hpp>
+#endif
 #include "frei0r.hpp"
 #include "frei0r/math.h"
 
--- a/src/filter/facebl0r/facebl0r.cpp	2026-06-06 23:50:07.619218924 -0300
+++ b/src/filter/facebl0r/facebl0r.cpp	2026-06-06 23:50:07.620254699 -0300
@@ -19,6 +19,12 @@
 #include <stdio.h>
 #include <string.h>
 #include <opencv2/opencv.hpp>
+#if CV_VERSION_MAJOR >= 5
+// OpenCV 5 moved CascadeClassifier/HOGDescriptor from objdetect to the
+// contrib xobjdetect module; objdetect.hpp (pulled by opencv.hpp) no longer
+// declares them. https://github.com/opencv/opencv/ (4 -> 5 migration)
+#include <opencv2/xobjdetect.hpp>
+#endif
 #include "frei0r.hpp"
 #include "frei0r/math.h"