xwayland: Fix "-force-xrandr-emulation"

Commit 7cdcdfea introduced a new command line option
"-force-xrandr-emulation", however it is missing from the
ddxProcessArgument().

As a result, trying to use that command option would result in a error:

(EE) Unrecognized option: -force-xrandr-emulation

Make sure "-force-xrandr-emulation" is accounted for in Xwayland's
ddxProcessArgument().

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Fixes: 7cdcdfea - xwayland: Add -force-xrandr-emulation switch
5 jobs for fix-force-xrandr-emulation in 1 minute and 52 seconds (queued for 2 seconds)