package/ffmpeg: add optional dependency to libxcb

package/ffmpeg: add optional dependency to libxcb

ffmpeg 3.3 added optional support for libxcb:
https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/HEAD:/Changelog#l282

This patch ensures a reproducible build of libavdevice.so:

$ readelf -a output/target/usr/lib/libavdevice.so | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libavfilter.so.7]
 0x0000000000000001 (NEEDED)             Shared library: [libavformat.so.58]
 0x0000000000000001 (NEEDED)             Shared library: [libavcodec.so.58]
 0x0000000000000001 (NEEDED)             Shared library: [libavutil.so.56]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb-shm.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb-shape.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb-xfixes.so.0]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.0]

compared to

$ readelf -a output/target/usr/lib/libavdevice.so | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libavfilter.so.7]
 0x0000000000000001 (NEEDED)             Shared library: [libavformat.so.58]
 0x0000000000000001 (NEEDED)             Shared library: [libavcodec.so.58]
 0x0000000000000001 (NEEDED)             Shared library: [libavutil.so.56]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.0]

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>