xwayland: add a standalone recipe

Please see here for the rationale for splitting XWayland out of
main xserver-xorg tree:
https://fedoraproject.org/wiki/Changes/XwaylandStandalone

Release announcement:
https://lists.x.org/archives/xorg-announce/2021-March/003076.html



Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>