Fixed CMake issue with FFMPEG highgui configuration

Currently, FFMPEG source files are included in highgui project file regardless of CMake WITH_FFMPEG option.
After applying this PR FFMPEG files are included only if WITH_FFMPEG option is enabled.
Use FFMPEG capture only if HAVE_FFMPEG flag is defined.
Merge pull request #3261 from a-wi:CMake_FFMPEG_config