xen/9pfs: p9_trans_xen_init and p9_trans_xen_exit can be static

Fixes the following sparse warnings:

net/9p/trans_xen.c:528:5: warning:
 symbol 'p9_trans_xen_init' was not declared. Should it be static?
net/9p/trans_xen.c:540:6: warning:
 symbol 'p9_trans_xen_exit' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>