package/vlc: Add libsidplay2 as optional dependency

vlc has a sid plugin:

$ output/host/usr/bin/i586-buildroot-linux-uclibc-readelf -a
  output/target/usr/lib/vlc/plugins/demux/libsid_plugin.so |
  grep NEEDED | grep sid
 0x00000001 (NEEDED)     Shared library: [libsidplay2.so.1]
 0x00000001 (NEEDED)     Shared library: [libresid-builder.so.0]

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