The `CV_RGB` macro is now in `imgproc.hpp`.

Fixes:

    ../subprojects/gst-plugins-bad/ext/opencv/gsthanddetect.cpp:497:40: error: ‘CV_RGB’ was not declared in this scope
            cvCircle (img, center, radius, CV_RGB (0, 0, 200), 1, 8, 0);
                                        ^~~~~~