package/tvheadend: select libvpx/x264 for transcoding support

package/tvheadend: select libvpx/x264 for transcoding support

ffmpeg needs libvpx and x264 to support transcoding.

Fixes this error message from tvheadend log:
2016-05-14 22:52:21.962 [  ERROR]:transcode: 0006: Unable to find libx264 encoder
2016-05-14 22:52:21.962 [   INFO]:transcode: 0006: 1:H264 ==> Passthrough
2016-05-14 22:52:21.963 [   INFO]:transcode: 0006: 2:MPEG2AUDIO ==> AAC (aac)

or

2016-05-14 22:58:01.529 [  ERROR]:transcode: 0009: Unable to find libvpx encoder
2016-05-14 22:58:01.529 [   INFO]:transcode: 0009: 1:H264 ==> Passthrough
2016-05-14 22:58:01.529 [   INFO]:transcode: 0009: 2:MPEG2AUDIO ==> VORBIS (libvorbis)

With this patch transcoding is logged like this:

2016-05-16 20:14:56.515 [   INFO]:transcode: 0004: 1:TELETEXT ==> Filtered
2016-05-16 20:14:56.516 [   INFO]:transcode: 0004: 2:MPEG2VIDEO 720x576 ==> H264 640x512 (libx264)
2016-05-16 20:14:56.516 [   INFO]:transcode: 0004: 3:MPEG2AUDIO ==> AAC (aac)

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>