cmake: fix ffmpeg check code (port into master #7893)

3.2.0-rc

"install DIRECTORY does not allow "COMPONENT" after PATTERN or REGEX."

find_package allows to specify optional components. This way, the
command will not fail if any of the components marked as optional was
not found. This is useful in cases where components such as xfeatures2d,
viz etc. are not available either because they were not compiled by the
user or package maintainers decided to not provide the packages at all.

The user can check the availability of the optional component using the
OpenCV_<component>_FOUND variable.

Fix #4958 cv::ml::StatModel::calcError not working for responses of type CV_32S