package/vlc: add optional support for xcb-util-keysyms

package/vlc: add optional support for xcb-util-keysyms

When xcb-util-keysyms 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/control/libxcb_hotkeys_plugin.so | grep NEEDED | sort
[...]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb-keysyms.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libxcb.so.1]
 0x0000000000000001 (NEEDED)             Shared library: [libXdmcp.so.6]

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