package/vlc: Add optional dependency to libtaglib

taglib is an optional dependency to vlc:
http://git.videolan.org/gitweb.cgi?p=vlc.git;a=blob;f=configure.ac;h=b9bd76a829da8ee8b8ea69a141827abd87c47cb2;hb=HEAD#l4020

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a
  output/target/usr/lib/vlc/plugins/meta_engine/libtaglib_plugin.so
  | grep NEEDED
 0x00000001 (NEEDED)                     Shared library: [libvlccore.so.8]
 0x00000001 (NEEDED)                     Shared library: [librt.so.1]
 0x00000001 (NEEDED)                     Shared library: [libpthread.so.1]
 0x00000001 (NEEDED)                     Shared library: [libdl.so.1]
 0x00000001 (NEEDED)                     Shared library: [libtag.so.1]
 0x00000001 (NEEDED)                     Shared library: [libz.so.1]
 0x00000001 (NEEDED)                     Shared library: [libstdc++.so.6]
 0x00000001 (NEEDED)                     Shared library: [libm.so.1]
 0x00000001 (NEEDED)                     Shared library: [libc.so.1]
 0x00000001 (NEEDED)                     Shared library: [libgcc_s.so.1]

Add it to vlc.mk to get reproducable builds.

For taglib itself zlib is only an optional dependency
http://git.buildroot.net/buildroot/tree/package/taglib/taglib.mk#n13

But taglib support in vlc depends on zlib:
http://git.videolan.org/gitweb.cgi?p=vlc.git;a=blob;f=modules/meta_engine/Makefile.am;h=003d292cdc9a80762a6515e0ced77552f13b5120;hb=HEAD#l10


so we select BR2_PACKAGE_ZLIB if BR2_PACKAGE_TAGLIB is selected.

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