xenbus: add module description

Modules without a description now cause a warning:

WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/xen/xenbus/xenbus_probe_frontend.o

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250328113302.2632353-1-arnd@kernel.org>