package/vlc: Add optional dependency to zlib

zlib is an optional dependency of vlc:
http://git.videolan.org/gitweb.cgi?p=vlc.git;a=blob;f=configure.ac;h=d44151b3fb507141ce7f9b98e95fa11edcdaac01;hb=HEAD#l811

Thanks to Yann for pointing this out:
http://patchwork.ozlabs.org/patch/529248/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>