package/x11r7/xlib_libXfixes: bump version to 6.0.0

Release notes:
https://lists.x.org/archives/xorg-announce/2021-May/003086.html



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>