arm64/xen: Fix too few arguments to function grub_create_loader_cmdline()

arm64/xen: Fix too few arguments to function grub_create_loader_cmdline()

Without this fix, building xen_boot.c omits:

loader/arm64/xen_boot.c: In function ‘xen_boot_binary_load’:
loader/arm64/xen_boot.c:370:7: error: too few arguments to function ‘grub_create_loader_cmdline’
       grub_create_loader_cmdline (argc - 1, argv + 1, binary->cmdline,
       ^~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from loader/arm64/xen_boot.c:36:0:
../include/grub/lib/cmdline.h:29:12: note: declared here
 grub_err_t grub_create_loader_cmdline (int argc, char *argv[], char *buf,

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>