Add av component (ffmpeg)

[Tobias-Fischer]

Currently the builds are missing the av component:

-- Checking for module 'libswscale'
--   No package 'libswscale' found
-- Could NOT find SWSCALE (missing: SWSCALE_FOUND) 
-- Looking for libswscale - not found

-- Checking for module 'libavdevice >= 56.4.100'
--   No package 'libavdevice' found
-- Could NOT find AVDEVICE (missing: AVDEVICE_FOUND) (Required is at least version "56.4.100")
-- Looking for libavdevice - not found

-- Checking for module 'libavformat'
--   No package 'libavformat' found
-- Could NOT find AVFORMAT (missing: AVFORMAT_FOUND) 
-- Looking for libavformat - not found

-- Checking for module 'libavcodec'
--   No package 'libavcodec' found
-- Could NOT find AVCODEC (missing: AVCODEC_FOUND) 
-- Looking for libavcodec - not found

-- Checking for module 'libavutil'
--   No package 'libavutil' found
-- Could NOT find AVUTIL (missing: AVUTIL_FOUND) 
-- Looking for libavutil - not found

-- BUILD WARNINGS
-- 	Cannot build component [av] - Missing: libswscale
-- 	Cannot build component [av] - Missing: libavdevice
-- 	Cannot build component [av] - Missing: libavformat
-- 	Cannot build component [av] - Missing: libavcodec
-- 	Cannot build component [av] - Missing: libavutil
-- END BUILD WARNINGS

/cc @traversaro

[Tobias-Fischer]

Fixed with #14