xen/9pfs: select CONFIG_XEN_XENBUS_FRONTEND

All Xen frontends need to select this symbol to avoid a link error:

net/built-in.o: In function `p9_trans_xen_init':
:(.text+0x149e9c): undefined reference to `__xenbus_register_frontend'

Fixes: d4b40a02f837 ("xen/9pfs: build 9pfs Xen transport driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>