xbmc: add host-yasm dependency for MMX/64bit archs

Fixes error during xbmc configure:

yasm not found, use --disable-yasm for a crippled build

If you think configure made a mistake, make sure you are using the latest
version from Git.  If the latest version fails, report the problem to the
ffmpeg-user@ffmpeg.org mailing list or IRC #ffmpeg on irc.freenode.net.
Include the log file "config.log" produced by configure as this will help
solving the problem.
configure: error: Submodule lib/ffmpeg failed to configure

This will be needed for 13.0 Gotham.

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