9p: add missing module license for xen transport

The 9P of Xen module is missing required license and module information.
See https://bugzilla.kernel.org/show_bug.cgi?id=198109



Reported-by: Alan Bartlett <ajb@elrepo.org>
Fixes: 868eb122 ("xen/9pfs: introduce Xen 9pfs transport driver")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>