package/vlc: add optional support for libvncserver

package/vlc: add optional support for libvncserver

When libvncserver was compiled before, vlc will use it as optional
dependency:

$ output/host/usr/bin/x86_64-linux-readelf -a output/target/usr/lib/vlc/plugins/access/libvnc_plugin.so | grep NEEDED
 0x0000000000000001 (NEEDED)             Shared library: [libvlccore.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [librt.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libintl.so.8]
 0x0000000000000001 (NEEDED)             Shared library: [libdl.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libvncclient.so.0]
[...]

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