vlc: add optional dependency to libmatroska

libmatroska is an optional dependency to vlc:
http://git.videolan.org/?p=vlc.git;a=blob;f=configure.ac;h=c46120d7f2a7d49d68d488d617e071148b8c5595;hb=HEAD#l2044



Add it to vlc.mk to get reproducable builds.

Signed-off-by: Pieterjan Camerlynck <pieterjan.camerlynck@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>