(cherry picked from commit 2b2e3ee9)

Conflicts:
	cmake/OpenCVFindLibsVideo.cmake
	modules/videoio/src/cap.cpp
	modules/videoio/src/cap_ffmpeg_impl.hpp
	modules/videoio/src/ffmpeg_codecs.hpp
	modules/videoio/test/test_ffmpeg.cpp

While the FFmpeg video IO backend gets compiled even without it,
the tests (for both FFmpeg and video IO) don't. This should fix
that.